Well, there ARE Colt .25 automatics marked UNITED STATES PROPERTY or U.S. PROPERTY. And people do pay more for them.
The question is whether they were marked that way at the factory, by some government agency, or by postwar entrepreneurs. My money is on the latter. Pate mentions that several were made up by a collector (unnamed) and given to some friends as a joke. Any of those would be marked, but none would be (IMO) valid collector's items.
The trouble with that gun is simply that no one knows for sure. There are no records that say such markings were ever applied legitimately, but (as usual in trying to prove a negative) no proof that they weren't. Until I see better evidence for an official marking, I will stick with my opinion. (Yes, I know, everybody has one.)
Another opinion, FWIW - if anyone found a .25 Colt which could be shown to be a genuine PROPERTY marked, the premium would be a lot greater than the 100% the Blue Book shows.
P.S. In my above posting, "after the way" should read "after the war." Sorry.