I was in Viet Nam between 1965-1966. It was common to go out on 20 to 30 day patrols with about a week in between to rest, clean up and go out again.
I spent three solid months in the bush on one op.
During that time, my M14 was locked and loaded, safety on, my 1911 .45 was locked and loaded, safety on.
I don't know about all of my buddies, but I suspect they did the absolute same.
Flipping the safety on an M14 was silent and the reason it was carried on safe was because of the design of it. It could be manipulated either way by wait-a-minute vines and even equipment on my TA-50.
Keeping it on safe was a mind-thing, it was something I did that I knew I had to do to fire.
I know that sounds funny, but on or off safe, my fingers touched it and operated so smoothly, I knew no other way. If you are familiar with the M-14, you will know what I mean.
As for the 1911, it was so worn that if off safe with hammer back, it only had to be breathed on to fire.
It was a noise maker, for all practical purposes, put together by the unit armorer from pieces of other .45's.
A Remington receiver (the typewriter company, not the gun maker), a Colt slide, God knows what else.
But, I digress.
Long story short:
The powers that be could not and did not even try, to dictate a standard carry method.
The only time I had a problem was in the Naval NCO Club in Danang, from some REMF that got all ballistic over the .38 Colt I had in a shoulder holster.
He simply could not believe I had rounds in the weapon and was in his club.
Never got far enough south to test the waters, so to speak, but did fly into Tan Son Nhut, once on a security mission.
We were bussed from there to Plantation, to the VIP escort helicopter company and bussed back.
Nobody said doo-doo to us, and I suspect it was because we looked like porcupines, we had so many weapons barrels sticking out from us.
I'd tell you what we were up to, except I'd have to kill you then.
Ahhh...the memories!