They changed the aluminum part where it bolts to engine. Early is a sort of cup, late more of a flat plate. Parts book shows same fields and armature, so presumably no change in function.
They interchange freely on 221-239-255 engines except for the automatic transmission ones, and V8 60 is different as well.
The differences between engagement depth of flywheel ring gear between early and late are handled at the starter mounting and so not a barrier to starter interchange.