This same topic is going on over on the Ruger forum, just thought I would add there's some good info over there about Ruger Blackhawks being used quite a bit as "self issued" sidearms in Vietnam, including some links and info about pictures of guys in Vietnam carrying them.
Maybe not Colt SAA's, but the Blackhawks are loosely based on the SAA and I was pretty interested to hear about guys carrying single action .357's and .44's in Vietnam.
I would bet Vietnam was the last confict where things were "loose" and no one much really cared about what guys had sent from home or brought with them. As long as you maintained your "issue" weapon, M16, M60 etc. I don't think a CO or senior NCO was going to care if a guy had a revolver that came in a care package stuffed in his belt.
During my time overseas in Iraq, from 2007-08 the closest I came to a "sidearm" was a CZ70 .32 ACP I found in a known insurgent's house, hidden in a bookshelf. I stuffed it inside my IBA, in the flap that held the kevlar, and kept it hidden there and carried it for like 2 weeks on missions without telling anyone......and then decided the odds of me needing a little .32 pop gun to defend myself did not outweigh the punishment of being caught...I don't know what would have happened if my squad leader would have found it during a PCI, probably would have told me to get rid of it but I wasn't looking to find out...so I ended up tossing it down an open sewer when no one was looking.....I later found a few more pistols, another CZ-50 or 70, I don't remember, a Tariq pistol,lots of Glock 19's, a rusted busted Hi Power but didn't take any more risks by taking any of those. One thing that stood out was I was searching a house and found a drawer full of old .38 S&W rounds.......being a"gun guy" I knew what they were and figured there was probably a Webley or Enfield revolver around somewhere, maybe even a Victory but never found it.