what the difference between B/RB lifter's part# 812 vs 976, they come as the 812 seems to be pre 1968 & the 976 after 68 what is the diff.

Mopar used a 1/4 ball cup in the lifters with a 5/16 ball cup in the rocker arms necessitating a 1/4 ball and 5/16 ball pushrod, the cup height was lower also than the next lifters they started using in 1967/1968 that had 5/15 cup in them so Mopar could use a 5/16x5/16 ball and ball pushrod that the factory guys could install either end were they wanted to put them instead of having to put the narrow end in the lifters

The changeover year was '68.

That explains it thanks guys. I knew Cab would chime in, I was looking at a NOS sealed power HT812 set & when I ran the #s for 71 that # didn't come up, knowing how good quality lifters are getting harder to come buy.