They are basically the same...
The series 75's have a machined head so it is shiney...and the earlier ones have a cast appearance with the same (bead blasted? ) finish as the case.
I think most, if not all of the earlier ones also have a "stuffer" crank...(that's what I have heard it called ) where the counter weight looks like a completely round disc when viewed through the backplate opening, and it has a alum. ring shrunk around the outside of it.

AFAIK the pistons, sleeves, rods, etc. are the same...and all the #8011 parts will exchange.
The early "series 75" also had slotted (4-40 ) screws...I don't know exactly when they went to socket head cap screws...late 70's I think.