From what I remember the really early 70’s had a different piston seal which is now obsolete. I had a 1976 version and Weihrauch informed me that in order to get a new seal I would need to purchase a replacement piston as the later pistons use the usual 25mm closed face synthetic type seals.
I also vaguely remember that Weihrauch changed something on the cylinder end cap I can’t quite remember what they did but it was something to do with the end cap security on earlier models.
AFAIK the triggers are the same and as you mentioned the grips have changed material wasn’t it Bakelite originally?
I vaguely remember something about the safety catch as well.