"It was the issue of hearing protection in particular that made me suppose there were indeed other firearms than the 1911 involved,"
In 2007 I knew of a unit doing live fire exercises in a block basement without hearing protection. With M4s. I doubt they worried at all in Vietnam.
I'd imagine suppressors didn't do well if dragged through the mud as described above.
I am also a little surprised they didn't have the Nagant available. Had to be a few of them in US possession and as overcomplicated as the design is it is a lot simpler and reliable than some of the modified revolvers described above. Of course, the 7.62 Nagant round doesn't compare to a 44 mag and probably doesn't outpower the 25 ACP by much.