For that model, probably nothing. That will be a transitiona with many features of the current 75b guns, and will probably use current magazines, too.
Does it have the "squared" trigger guard? Probably has the newer sights, too. (Check for a length-wise dovetail on the front sight.)
Not as pretty as the classic pre-Bs, but probably a more practical gun.
(Mags are about the only issue with the pre-B, as only stock, factory 10 and 15 rounders fit. On the very oldest ones, some parts are getting rare, including parts associated with the safety which has an internal spring. Newer models don't have that. Look for a pinhole in the safety lever to know you've got an older one.)