I'm going to forget some details, but he was not using hobby level CNC products. I still have a Kollmorgen AKD motion controller that he sent me as a test bench. Most of the old tools were using 0-10V sawtooth analog outputs for motor position and, IIRC, their preferred controllers didn't support that. The initial version of the box I made took that 0-10V and pretended to be a resolver so the AKD could control the motor.

Like I said, I don't remember all his design decisions, but they wanted to retain the original position sensors.