Hard to tell as is... (even with the "M")

If you can figure out what thread the frame has, that would be a huge help. Also, tube size and total weight will help things way more

Based on things like the location of the shifters, the fact that there are holes for 2 watercages drilled, plus that hexagonal nut, it is very unlikely that this thing started life as an italian bike. I would guess French, American or Japanese from '75 to early '90s. The lugs/headset look a tad generic, so I would suspect that it is not a terribly high end bike. If it weighs a ton, "M" could stand for "Murray"