diff --git a/selfhost/cmd/w6c/main.combined.ww b/selfhost/cmd/w6c/main.combined.ww
index 2ab930f6..b865e53c 100644
--- a/selfhost/cmd/w6c/main.combined.ww
+++ b/selfhost/cmd/w6c/main.combined.ww
@@ -25018,6 +25018,34 @@ fn aggcopy(c: *cgen, sz: i32) void = {
 	};
 };
 
+// copysrcnatsize — natural byte width of a whole-struct copy's SOURCE
+// operand, read from the source node's stamped tinfo (tichase peels
+// NAMED). #71: the four whole-struct field-copy sites must move this many
+// bytes, NOT structinfo.totsize (slot-padded, round-8) which over-copies
+// into slot padding and clobbers a natural-offset successor once #44 packs
+// it there. Reads the source NODE's type, never fi.foff/fi.fsz or
+// structinfo, so #71 stays independent of #44's registerstruct offset
+// change. Equals cstage's `str_fu->size` (cmd/w6c/cgen.c:5302 SSoT) — the
+// field struct's aligned r.size (check.ww N_TSTRUCT), distinct from the
+// existing structnaturalsize (which reads structinfo max(foff+fsz), a
+// #44-coupled source). The ragged-tail completeness shared by both stages'
+// field copies is a separate class, tracked under #73 / the aggcopy
+// emitter choke-point.
+fn copysrcnatsize(c: *cgen, src: *node) i32 = {
+	if (src == nil || src.type_ == nil) {
+		let msg: str = "#71: whole-struct copy source has no stamped tinfo\n";
+		os.write(2, msg.ptr, msg.len: u64);
+		os.exit(1);
+	};
+	let ti: *tinfo = tichase(src.type_: *tinfo);
+	if (ti == nil) {
+		let msg: str = "#71: whole-struct copy source tinfo chase yielded nil\n";
+		os.write(2, msg.ptr, msg.len: u64);
+		os.exit(1);
+	};
+	return ti.size: i32;
+};
+
 // cgplaceaddr — compute the ADDRESS of an arbitrary place (lvalue)
 // expression into dstreg; returns true when the shape is wired, false
 // otherwise (the caller loud-stops — rule 7, never a silent drop).
@@ -33259,7 +33287,7 @@ fn cgassign(c: *cgen, n: *node) void = {
 													emitline("\tMOVQ\t");
 													emitoff(lc.off: i64);
 													emitline("(BP), BX\n");
-													let ssz: i32 = ssi.totsize;
+													let ssz: i32 = copysrcnatsize(c, n.rhs);	// #71: natural source size, not slot-padded totsize
 													let k: i32 = 0;
 													for (k + 8 <= ssz) {
 														emitline("\tMOVQ\t");
@@ -33485,7 +33513,7 @@ fn cgassign(c: *cgen, n: *node) void = {
 												let ssi: *structinfo = structlookup(c, fi.tnode.str);
 												let srhs: *local = localfindnode(c, n.rhs.str);
 												if (ssi != nil) { if (srhs != nil) {
-													let ssz: i32 = ssi.totsize;
+													let ssz: i32 = copysrcnatsize(c, n.rhs);	// #71: natural source size, not slot-padded totsize
 													let k: i32 = 0;
 													for (k + 8 <= ssz) {
 														emitline("\tMOVQ\t");
@@ -33772,7 +33800,7 @@ fn cgassign(c: *cgen, n: *node) void = {
 											emitline("\tLEAQ\t");
 											emitsymname(c, bn);
 											emitline("(SB), BX\n");
-											let ssz: i32 = ssi.totsize;
+											let ssz: i32 = copysrcnatsize(c, n.rhs);	// #71: natural source size, not slot-padded totsize
 											let k: i32 = 0;
 											for (k + 8 <= ssz) {
 												emitline("\tMOVQ\t");
@@ -34415,7 +34443,7 @@ fn cgassign(c: *cgen, n: *node) void = {
 								emitline("(SB), CX\n");
 							};
 						};
-						let ssz: i32 = ssi.totsize;
+						let ssz: i32 = copysrcnatsize(c, n.rhs);	// #71: natural source size, not slot-padded totsize
 						let k: i32 = 0;
 						for (k + 8 <= ssz) {
 							emitline("\tMOVQ\t");
diff --git a/selfhost/cmd/wcc/cgenexpr.ww b/selfhost/cmd/wcc/cgenexpr.ww
index 326dd1c8..32eff84b 100644
--- a/selfhost/cmd/wcc/cgenexpr.ww
+++ b/selfhost/cmd/wcc/cgenexpr.ww
@@ -1699,6 +1699,34 @@ fn aggcopy(c: *cgen, sz: i32) void = {
 	};
 };
 
+// copysrcnatsize — natural byte width of a whole-struct copy's SOURCE
+// operand, read from the source node's stamped tinfo (tichase peels
+// NAMED). #71: the four whole-struct field-copy sites must move this many
+// bytes, NOT structinfo.totsize (slot-padded, round-8) which over-copies
+// into slot padding and clobbers a natural-offset successor once #44 packs
+// it there. Reads the source NODE's type, never fi.foff/fi.fsz or
+// structinfo, so #71 stays independent of #44's registerstruct offset
+// change. Equals cstage's `str_fu->size` (cmd/w6c/cgen.c:5302 SSoT) — the
+// field struct's aligned r.size (check.ww N_TSTRUCT), distinct from the
+// existing structnaturalsize (which reads structinfo max(foff+fsz), a
+// #44-coupled source). The ragged-tail completeness shared by both stages'
+// field copies is a separate class, tracked under #73 / the aggcopy
+// emitter choke-point.
+fn copysrcnatsize(c: *cgen, src: *node) i32 = {
+	if (src == nil || src.type_ == nil) {
+		let msg: str = "#71: whole-struct copy source has no stamped tinfo\n";
+		os.write(2, msg.ptr, msg.len: u64);
+		os.exit(1);
+	};
+	let ti: *tinfo = tichase(src.type_: *tinfo);
+	if (ti == nil) {
+		let msg: str = "#71: whole-struct copy source tinfo chase yielded nil\n";
+		os.write(2, msg.ptr, msg.len: u64);
+		os.exit(1);
+	};
+	return ti.size: i32;
+};
+
 // cgplaceaddr — compute the ADDRESS of an arbitrary place (lvalue)
 // expression into dstreg; returns true when the shape is wired, false
 // otherwise (the caller loud-stops — rule 7, never a silent drop).
@@ -9940,7 +9968,7 @@ fn cgassign(c: *cgen, n: *node) void = {
 													emitline("\tMOVQ\t");
 													emitoff(lc.off: i64);
 													emitline("(BP), BX\n");
-													let ssz: i32 = ssi.totsize;
+													let ssz: i32 = copysrcnatsize(c, n.rhs);	// #71: natural source size, not slot-padded totsize
 													let k: i32 = 0;
 													for (k + 8 <= ssz) {
 														emitline("\tMOVQ\t");
@@ -10166,7 +10194,7 @@ fn cgassign(c: *cgen, n: *node) void = {
 												let ssi: *structinfo = structlookup(c, fi.tnode.str);
 												let srhs: *local = localfindnode(c, n.rhs.str);
 												if (ssi != nil) { if (srhs != nil) {
-													let ssz: i32 = ssi.totsize;
+													let ssz: i32 = copysrcnatsize(c, n.rhs);	// #71: natural source size, not slot-padded totsize
 													let k: i32 = 0;
 													for (k + 8 <= ssz) {
 														emitline("\tMOVQ\t");
@@ -10453,7 +10481,7 @@ fn cgassign(c: *cgen, n: *node) void = {
 											emitline("\tLEAQ\t");
 											emitsymname(c, bn);
 											emitline("(SB), BX\n");
-											let ssz: i32 = ssi.totsize;
+											let ssz: i32 = copysrcnatsize(c, n.rhs);	// #71: natural source size, not slot-padded totsize
 											let k: i32 = 0;
 											for (k + 8 <= ssz) {
 												emitline("\tMOVQ\t");
@@ -11096,7 +11124,7 @@ fn cgassign(c: *cgen, n: *node) void = {
 								emitline("(SB), CX\n");
 							};
 						};
-						let ssz: i32 = ssi.totsize;
+						let ssz: i32 = copysrcnatsize(c, n.rhs);	// #71: natural source size, not slot-padded totsize
 						let k: i32 = 0;
 						for (k + 8 <= ssz) {
 							emitline("\tMOVQ\t");
diff --git a/selfhost/cmd/wwdump/main.combined.ww b/selfhost/cmd/wwdump/main.combined.ww
index a1dd4560..d4e5cc1c 100644
--- a/selfhost/cmd/wwdump/main.combined.ww
+++ b/selfhost/cmd/wwdump/main.combined.ww
@@ -25018,6 +25018,34 @@ fn aggcopy(c: *cgen, sz: i32) void = {
 	};
 };
 
+// copysrcnatsize — natural byte width of a whole-struct copy's SOURCE
+// operand, read from the source node's stamped tinfo (tichase peels
+// NAMED). #71: the four whole-struct field-copy sites must move this many
+// bytes, NOT structinfo.totsize (slot-padded, round-8) which over-copies
+// into slot padding and clobbers a natural-offset successor once #44 packs
+// it there. Reads the source NODE's type, never fi.foff/fi.fsz or
+// structinfo, so #71 stays independent of #44's registerstruct offset
+// change. Equals cstage's `str_fu->size` (cmd/w6c/cgen.c:5302 SSoT) — the
+// field struct's aligned r.size (check.ww N_TSTRUCT), distinct from the
+// existing structnaturalsize (which reads structinfo max(foff+fsz), a
+// #44-coupled source). The ragged-tail completeness shared by both stages'
+// field copies is a separate class, tracked under #73 / the aggcopy
+// emitter choke-point.
+fn copysrcnatsize(c: *cgen, src: *node) i32 = {
+	if (src == nil || src.type_ == nil) {
+		let msg: str = "#71: whole-struct copy source has no stamped tinfo\n";
+		os.write(2, msg.ptr, msg.len: u64);
+		os.exit(1);
+	};
+	let ti: *tinfo = tichase(src.type_: *tinfo);
+	if (ti == nil) {
+		let msg: str = "#71: whole-struct copy source tinfo chase yielded nil\n";
+		os.write(2, msg.ptr, msg.len: u64);
+		os.exit(1);
+	};
+	return ti.size: i32;
+};
+
 // cgplaceaddr — compute the ADDRESS of an arbitrary place (lvalue)
 // expression into dstreg; returns true when the shape is wired, false
 // otherwise (the caller loud-stops — rule 7, never a silent drop).
@@ -33259,7 +33287,7 @@ fn cgassign(c: *cgen, n: *node) void = {
 													emitline("\tMOVQ\t");
 													emitoff(lc.off: i64);
 													emitline("(BP), BX\n");
-													let ssz: i32 = ssi.totsize;
+													let ssz: i32 = copysrcnatsize(c, n.rhs);	// #71: natural source size, not slot-padded totsize
 													let k: i32 = 0;
 													for (k + 8 <= ssz) {
 														emitline("\tMOVQ\t");
@@ -33485,7 +33513,7 @@ fn cgassign(c: *cgen, n: *node) void = {
 												let ssi: *structinfo = structlookup(c, fi.tnode.str);
 												let srhs: *local = localfindnode(c, n.rhs.str);
 												if (ssi != nil) { if (srhs != nil) {
-													let ssz: i32 = ssi.totsize;
+													let ssz: i32 = copysrcnatsize(c, n.rhs);	// #71: natural source size, not slot-padded totsize
 													let k: i32 = 0;
 													for (k + 8 <= ssz) {
 														emitline("\tMOVQ\t");
@@ -33772,7 +33800,7 @@ fn cgassign(c: *cgen, n: *node) void = {
 											emitline("\tLEAQ\t");
 											emitsymname(c, bn);
 											emitline("(SB), BX\n");
-											let ssz: i32 = ssi.totsize;
+											let ssz: i32 = copysrcnatsize(c, n.rhs);	// #71: natural source size, not slot-padded totsize
 											let k: i32 = 0;
 											for (k + 8 <= ssz) {
 												emitline("\tMOVQ\t");
@@ -34415,7 +34443,7 @@ fn cgassign(c: *cgen, n: *node) void = {
 								emitline("(SB), CX\n");
 							};
 						};
-						let ssz: i32 = ssi.totsize;
+						let ssz: i32 = copysrcnatsize(c, n.rhs);	// #71: natural source size, not slot-padded totsize
 						let k: i32 = 0;
 						for (k + 8 <= ssz) {
 							emitline("\tMOVQ\t");