Not entirely sure. I've tried not to handle it too much. But I also don't know what to look for. I don't believe I've seen a US stamp on it though. Those pictures were sent to me by my dad this past summer. I may take a closer look and see what I can find. I know I've found the serial numbers. Guessing we could find out exactly what it is from that? Or at least manufacturing date?
Also the holster shown in the photo was made from a Japanese Briefcase. Guessing by my grandpa.
Usually marked "US Property" under the barrel and "US ARMY Mod 1917" on butt by the lanyard loop. I have one in original blue and one rearsenaled and parkerized in WWII. The 45ACP uses half moon clips while the New Service in 45 Colt loads like any other revolver.