All known US&S pistols were finished with the DuLite bluing process, which presents a thinned and frequently irregular appearance. Based on the OP's photo I would suspect that this may be the original finish that has worn over time. WW2 contracts specified the Parkerizing process, but wartime exigencies led to acceptance of the non-standard pistols to fill essential needs.
Many issued pistols will have some mismatched parts. This was normal for unit maintenance procedures. If the pistol were ever sent for arsenal re-work it would most likely have been Parkerized and mismatching of parts would be all over the board.
US&S subcontracted several parts including barrels, magazines, some springs, and others. This was common among the various producers of WW2 era, and cooperation was encouraged between the various companies to facilitate war-time production efforts.
All US&S pistols are of interest to collectors due to the relatively low production numbers. US&S had some serious labor problems during the contract period, missing a few payrolls and causing quite a bit of animosity among the workforce. More than a few pistols disappeared from the factory, presumably taken by workers who resented not being paid on time. One of my US&S pistols came from the estate of the son of a man who worked at the US&S plant during that time and proudly displayed that pistol as proof that he had gotten something for his time and skills while many others had not received anything.
Union Switch & Signal Corporation was a manufacturer of equipment for railroad applications (switching and signaling gear), based in Swissvale, Pennsylvania.
