I know I've seen an ad for these with a Bulova movement. I can't find it right now, but it was one of these really early ones that I think advertised other brands along with Bulova to fit in this case.
Really neat piece... octogonal, 17 Jewels. I don't know what the big "M" stands for on the movement. Could that be an early attempt at a movement model number?
If you look at the left side of the picture of the case back you'll see it got one of it's corners cut/bent. It doesn't look like it's "filled", but more solid. See what you think....
What do you think panel?