360 main bearing differences by years?

I have a 72 360. Went to by main bearings and discovered that the bearings for the mains are listed by year. 71-73 then 74 and up.

I have searched on here and google and can't figure out the difference. Can someone enlighten me? (The 74 and up bearings everyone has in stock. The 71-73 no such luck.)


Thanks,
Dan...