With respect to everyone here, I would like to point out that Kevin's site is primarily for Mustangs. His info about the slot in the 28 spline shaft is referenced to only cars, which is what I typed in post #6, and in this case, was probably not real helpful information. Both of my car (not mustangs) rears have the slot, but also have a 5x4.5" BP, so maybe the slot is more of an indication it is a smaller BP, than 28 or 31 splines in the case of seeing a 9" rear sitting alone without any car to ID its origin. This would hold true in Kevin's site, since Mustang's share the same BP. As for truck 9" rears, yes it appears they all have the 2 holes, opposite, and one smaller hole in the center. My '66 Bronco has it, my '68 donor for '48 has it, and 6 more 9" rears I have all have it. I can bet they all aren't 31 splines. So mcdonaldm would be right. But they all have 5x5.5" BP. The 9 3/8" of the trucks would also have this BP. I believe the Lincolns had 5x4.5"? So what it boils down to is, you can't tell by looking at the truck, unless you crawl underneath to look at the curved rib. I forget you all may have access to seeing the underside. Where I live, all the junkyard vehicles are stuffed into MN red clay where they sink over time, and the underside isn't viewable. Kevin's site is great, but it's oriented towards cars, not all 9" rears. My $.02 ok, I need coffee now.