Active manufacturers: Rapoo, Nappoo
Non-active manufacturers: IBM, Univac/Unisys, DEC, Amdahl... basically any big computer firm made their own keyboards.
Early computers didn't have industry standards (apart from IBM's 80-column card) as far as I know, so each computer could only use the keyboard it shipped with or some very similar models from the same manufacturer.
If you plug this:
into this:
Both will probably explode in a cloud of expensive fireworks.
Incidentally, the bank of switches on the control panel to the lady's right is actually the "keyboard" of the IBM 709. You use it to input a 32-bit binary word into a selected memory address, which probably means it has the most direct input method ever, directly editing a memory location. I wonder if that is considered "mechanical" enough...