I agree, it would be nice to have a 10mm USP compact, but it definitely would have to be on the .45 size frame. I'm curious as to the conversion process, including modification of the magazines.
I would take a guess that they used a .40 slide on the .45 frame and stuck a 10mm barrel on it with modified magazines. I don't see how they could do it otherwise with so much of a difference between the case head diameters of the .45 and .40.
Another solution to get 10mm performance is to get a 45 USP Compact and a barrel for one of the 45's necked down to 10mm, similar to the .40 necked down to make the .357 Sig. I think the cartridge is named .40 Cor Bon or something like that. There are several of the wildcats/hybrids floating around and is hard to keep up with them.
Good Shooting
Red