I think they were left over from earlier times, or were "smuggled" in by people who believed them superior for whatever reason. The 30-30 round of the carbine would have been a better leaf cutter by a little. The military .45 round was greatly over rated. I saw carbines, but not many. Often I think they may have been liberated from VC. The grease gun was fun to play with. However, the M-16 was superior in all respects for Vietnam and most other combat. That's true but hard to say for an old M-1 sharpshooter and afficionado.
I was not aware of that. They may have been in the inventory, but I would not have thought they were used for anything but arming indigenous troops in various parts of the world, or for special ops trying to disallow any connection with the USA. If you know different I would be interested in hearing more, because most troops from mid-60s were using M-16s.