I suspect that there is some variation in valves from one manufacturer to another. This includes the length of the stem and the type of keepers. When I was doing a valve adjustment on my BDA (story coming soon!), I bought some valve shims from Dave Bean. What I got was not the flat shims my engine had but little caps that fit on the top of the valve stem. The caps were deeper than the distance between the top of the keeper and the top of the valve stem. I didn't feel comfortable trimming the "skirt" of the cap enough to fit on my valves so I didn't use them but this leads me to believe there are at least variations in designs for a particular engine and there may in fact be generic valves that you choose based on a few variables like material, the stem diameter, stem length, and valve diameter.
All this is a fair amount of guessing and supposition so I hope someone with better knowledge of this will speak up.