That's the early system.

The spindle drive board (in your first picture post), should have everything marked on it.
As it's a NEControl, I'm not sure if a manual will be easily found, but some google searches might turn up something.

The only unknown will be what the inhibit (white/black wire) gets switched to. I'd trace that wire, and see if you can work out the circuit it connects to.
As it's connected to Inh (H), I suspect it'll get 5V applied to it to inhibit (disable) the drive, with 0V connected to the speed control L.

The grey and blue wires will go to a circuit that outputs 0-10V (or possibly 0-5V - you can measure the voltage between the L and H terminals, which will give you an idea) to set the spindle speed

Does the original computer power up?
If it does, it would be worth measuring the various voltages, as it'll help eliminate guess work.
It would also let you test the spindle drive, as SCR drives do have a habit of the SCRs failing causing the supply fuses to blow.

It's also worth measuring the voltages on the power supply board (Second photo).
If it's anything like the later board, it'll output ~24V for the stepper drivers, and 5V for the control, as well as providing a pass through for the spindle supply.

Although I'm not sure what the big plastic box does. Is that where the wires from the spindle drive board connect?
If so, it could the isolation circuit.