I will offer some info, not to step on Ed's thread. I'm sure he'll weigh in with some good info and correct me if my memory is faulty.
The 750 PBX is a small (3 by 20) crossbar exchange that has relays to operate the stations. There is no separate KSU.
The stations are not wired like standard 1A or 1A1 sets. They have very skinny mounting cords, and use a control lead per button, and can have a button for 3 CO lines and 2 intercom lines. The originals had no lamps, and visual indicators were mounted on the wall or the desk near the phones, so that users could see what lines were ringing or in use.
The 6-button sets used were the 462 (no lamps) and the 466 (with lamps.)
