metric nipple/cap size
I ran into this quite a few years back. Bought a brand new Uberti/Walker. Had nothing but problems with caps. Tried everything and nothing fit correctly. Took the pistol to a local BP expert & friend Bill Hammond of "Spring Brook Trading Post". He put a micrometer on the (cap end of the) nipples and said no wonder I was having problems the nipple was metric. In other words if I could get metric caps, they would work just fine. We changed the nipples and haven't had a problem since. He also informed me this is a fairly common problem with Italian reproductions.
Hope this answers the question!