Great write up, thanks. I was intrigued by the "tilt" keymapping on the last line of your posting. https://computerarcheology.com/Arcade/SpaceInvaders/ notes:
> In the early eighties you would have found the Space Invaders cabinet in an arcade right next to the pinball machines. So a "tilt" switch, like you would find in a pinball machine, would not have seemed as strange as it does today. If you shake, slap, or otherwise physically abuse an SI cabinet you will get a TILT message and your game will end.