I have a third member with casting numbers D54 and 513261. Is this a Old's third member? What years does it interchange with? The tooth count is 10-39. What ratio would that be?
Thanks!

 

I can't decode the number but it would be a 3.90 ratio. That will turn some RPM's on the highway.

 

I think it might only be a few years 54 55 56 ish or 53 54 55
We know 57-64 are the 9.3 and interchange from Pontiac and olds

I think your 54 has 10 spline / coarse spline axles and pontiac might have fine spline 16? Not sure about this info .

No posi rears