ok so I have 2 different posi rears. I bought one that I was told is 2.78. Got it home and then realized there's no brake lines or brakes etc so I bought another that had all the brake hardware and was told it's a 3.91 rear.
Fast forwards few years, I sell one online advertised as 2.78 posi. I double checked before loading into my truck and I think it's a 2.56 (2 turns of the drum gives me about 5-6 turns of yoke) so I check the other that's supposed to be 3.91 and get two drum rotations to about 8 turns if the yoke (2.78) Yes I know this isn't EXACT but it's close. The housing on the one I'm keeping doesn't have a 4 but it does have 0227 or whatever which is a later housing vs the other which had a different number making it older.
The older one said BS on axle tube if I remember correctly whereas the newer one says SJ.
The confusing thing is that I don't see BS listed anywhere and SJ is listed for older 60's rears. I see different ratios listed for it too.
What the heck do I have??!!! I thought I had this figured out when I got them but now I'm not so sure. So im
thinking the newer one with 0227 on the housing and SJ on axle tube is a 2.78 rear. Can anyone clarify? I think the older one was 31 spline and the newer one is 28. I dropping the older one off tomorrow 3.5 hrs away but I just want clarification with what I have. Want to keep everything olds.

Counting turns of the shaft is ok for a ballpark figure. It’s pretty hard to tell the difference between a 3.42 and 3.55 gearset just by a mark on the axle and turning the yoke. No way would I buy a supposedly desireable gear ratio advertised that way.