The Model 1917 Colts and S&Ws (Yes, same model number for both) were 'substitute standard' for the 1911 as well into WW2, both firing the same .45 ACP cartridge as the 1911, but using half-moon clips.
I have also seen the Colt 1903 in .32 ACP with 'U.S. Property' markings, which meant it was military issue, of course it would have been a senior officer's pistol. Pretty sure I recall that Patton had a rather customized one of them, whether it was originally government issue or private purchase I don't know.
I believe some time during the Viet Nam War era, DOD made it definitively illegal to take a personal weapon into a war zone. I certainly know of people who did in the earlier parts (Of course they may have been pushing the envelope a lot), but I've also heard from people that toward the end they were definitely not allowed to do it and the practice was heavily suppressed.