M-10
First and foremost, welcome to PS.Com. Not many One Zero's and One One's around and your experience is quite incredible.

Thank you for your service.
As you have no doubt noticed, there are some distinct differences between the gear carried today and the gear of the SF contingents in RVN. Most noticeably the turtle shell of body armor that we wear which gets in the way of almost anything attempted to be thrown over a shoulder.
Where many historical photos showed your RT's in a minimalist posture, we are not able to do the same. As such, many of us use a single point sling attachment on our weapon from the collapsible buttstock directly to our right (or left for the backwards folk) shoulder. This allows us a dynamic ready up while wearing the body armor without the fumblings of a sling. In those of us significantly more challenged in the coordination department, slings tend to get hooked on all sorts of goofy stuff (including my neanderthal type neck).
This single point sling attachment is relatively short, bringing the butt of the weapon quite close to the body. The cross body posture you see of many troops is due to the short distance of the buttstock attachment and the necessity to keep the weapon from flopping around. Therefor, it would either have to point straight down, straight out or off the left foot. Most choose the left foot.
I am sure there are other "more cool" (and perhaps more steely eyed killer) reasons - but this one is the one I tend to see as a matter of practicality.
Again - thank you for your service, and welcome to PS.Com