I don't think there is an answer to that question...at least not one I know. I've saved some caps to compare over the years and honestly I haven't been able to discern much difference (aside from the odd one with the pegs that had been set into it). If you want to use the neoprene replacement, first of all I'd look for an NOS Fel-Pro gasket on eBay. They created this seal and maybe Best Gasket is a wonderful operation but I think of this like I think of voltage regulators. There are the right ones and then there are all the imitators. I don't like re-doing rear seals, so I want to trust Fel-Pro in this case. If you put one in (sans gasket sealer) just check the fit as I mentioned with a piece of common copy paper by snugging up the rear bearing cap. If the engine is in the truck, check the top first and then the bottom and the sides. The crank will be sitting as low as it can be due to weight, gravity, etc so the top clearance will be the key. It is also important (as common sense should tell you) the closer to .001" clearance between your journal and bearing you have the better things will be back there. Good luck!
