There were 9 cover bolt versions with both small and large bolt axle patterns and then there is a large axle bolt pattern 10 bolt version that is very close to being a hybrid of the early axle and Dana 30 & 35 jeep axle parts. I would have to look it up again, but it seems that the 10 bolt cover came out with the 2-1/2 axle tubes swaged down to fit the 2 inch center section around '78 or so. Interchangeability is a given between 9 bolt units, but I have seen 10 bolt versions for F/J/M bodies, so it's difficult to say what internal differences you might find with them.
