One of the things that I appreciate about this forum is the depth of knowledge and experience that appears here. I half expected a retired RSM to weigh in on this topic. The last two film clips were interesting that there was no arm swing at "slow march". The 1931 clip has a couple of items of interest. One is the hand-cranked camera on the left side of the parade ground. The other is the sound quality- better than I'd expect for the early days of motion picture sound. The technology is really only 3-4 years old at the time. I'd suspect that the original has been cleaned up and rerecorded or the sound track was added later.