I'm just glad
someone appreciates this obscure side of the hobby!
My source on all this was a former ASCO employee that went on to help run Madaras in 1973. I showed him the "M" mark and he confirmed everything. I also asked about the equally mysterious "VET" mark we've discussed. Up to now, I was quite certain this was a concessionaire's mark used by ASCO during the early 1970s. So I inquired.... Unfortunately, he could not recall ever seeing it and could not identify its significance.
So there's still some mysteries left for us to solve.