The main difference so far I know is the use of a metell housing for the capsule up to the 80´s instead of the plastic housing in the new ones.
Old 421 ´s can sound very different. I checked a refurbished original without filter agains an old as it is original 421 and the difference was HUGE!!
The refurbished one was just AMAZING. Didn´t compare to new ones, but can imagine that the time sometimes didn´t passes by without doing some harm to a lot of old 421.
So you have to judge about the induvidual mic when it comes to vintage 421´s.