Sometimes, you simply have to know about the companies with which you are dealing. Initially, I think that some companies used metric near-misses for those things. However, at some time those companies (particularly Pedersoli and Uberti) became obsessed with making very accurate reproductions.
This concern for historical accuracy and detail has, on occasion, led me to pulling out hair. For example, I had an early Pedersoli Sharps and it was a nice rifle. However, when I went to put a tang peep sight on it, one of the screws supplied with the sight took off acrose the shop and I found that I needed a screw for mounting. Great, but it turned out to be the original thread of 10-28 that Sharps used. That is great for historical detail, but have you ever tried to get a 10-28 screw locally? Don't waste your time.
Well, I found a metric near-miss that would engage for about 4 threads before it bound up, so that got me through until one got here from the manufacturer.
BTW, Pedersoli copied Sharps' non-standard sight dovetails, too, and that was also a PITA.