Eutycus, some may not know but at least at one time there were differences between brands in cap sizes and even choices in the same brand of caps labelled as "10", "11", etc. Among my stashes of old things are a mixture of caps well over 100 years old. One difference is not only the diameter of the cap but also the length. I have some that are quite short - they work on a variety of pistol nipples the longer ones will not tho' they're the same diameter as the modern ones we see today. A too-long cap is bad ju-ju but will expose a flinch.
Goldmark is one that comes to mind among old ones, but also some Italian that were touted as "those hot Italian caps". You didn't have to do anything to the nipples, just had to find the right caps that fit a particular gun . We had choices back in the day. Old caps suck, too many duds.