Well, at one time Italian makers were all wrapped up in making things metric. Their copies were only reasonable facsimilies.
Then came the era of nearly exact reproduction. Most Uberti and Pedersoli guns are now as close a copy as they can make them--at least for small details like dovetails and screws. That is mostly possible, but not completely, as many of the old guns they used for copy work had production variations that were inherent in products made in a day and age where variation was the rule.
I have a Pedersoli Sharps rifle and it is great. However, it has a Sharps-dimesioned dovetail slot on the front sight and Sharps screw threads--neither of which are any easier to come by than their metric counterparts. Did you ever try to get a 10-28 screw or tap? They are scarce and expensive.
My Uberti revolvers sometimes have a mix of parts. For example, they may have Colt threads on the screws and then have metric threads on the nipples. Boy, isn't that nice?
I suggest that you do as I do: buy sets spares from the manufacturer and save the originals for the time when the gun is sold.