2nd generation steck parts have small bumps to fit tighter together, whereas 1st generation dont, and usually after a lot of playing they dont keep tiht anymore
are you sure? I did check on my 3445 prison tower I had for Christmas in 1977 and there are the "little bump". I bought few years ago the 3447 Rathaus boxed (the polystyrene is mint) with the instruction sheet and 100% complete for something like 40€...and there are also the "little bump"...what I want to say is that this guy who sold the 3447 didn't want to make money because of the price, and he had no other Playmobil to sell...
does the problem could be from the mold? may be the really first generation was with "little bump", with time "little bump" in the molds were damaged and they were change later, but some parts were made and put on the marketl with no "little bump"....when they did see the problem they changed the mold and so the "little bump" came back....
I say that but I'm really not sure, it's just an hypothesis...