Pull / look under the CP and see if there're any signs of button holes next to the joystick having been Bondoed at some point. It won't necessarily give a definitive answer, but will at least provide some clues.
One detail about this machine is driving me crazy: the serial number is stamped into the upper left corner on the rear.
That really does look like a Midway serial number stamping, but it's been a long time since I had one of these cabinets so my memory may not be the greatest. That said, I want to say that at some point with these cabinets they stopped stamping them and went to just attaching tags, but I don't remember which game would have started that or even if my memory is correct.
It looks almost as though there's a date of 22-10-81 written on the panel above the monitor. That would be an odd date format for someone to write on a US-production cabinet, but assuming that that means the 22nd of October, 1981, and it was written somewhere on the assembly line, it would put this machine right in the middle of the production runs for Bosconian, Galaga, Ms. Pac-Man, and Rally-X. Than again, it may mean nothing.
I kinda want to say it looks like a Galaxian with a Pac-Man control panel on it, but it's hard to say for certain. Any useful tags inside the machine?