The parts book had a different part number for the Big Block versus small block TC oil seal vintage 1970? What is the difference? Current seals seem made to fit either.

Nobody knows?

Me recollection is they are the same but the B/RB had a flange that limited how far it would go in the cover while the A was just a “bell” type.

Originally they were different but eventually someone at Chrysler was smart enough to figure out that they could be the same part. There are lots of examples of stuff like this. Chrysler Engineering was very poor at reuse and simplification during the 60's and 70's but gradually they started to figure things out and they worked to consolidate part numbers over time.

Ok thanks. Going thru a box of these in original Mopar parts box. I thought might be a subtle width difference. These are all flanged. There are a few examples that are much wider, but that seems to be a late 50s early 60 seal from what I see on ebay, so I guess a different timing cover in that time frame.