I needed a new master cylinder and found this thread with the search. I have the factory 4 piston disc set up. Based on the above post I ordered a master cylinder for a '73 Duster 340 with power disc brakes. Its a Raybestos Part Number MC36406 which is a new "Professional Grade".
www.rockauto.com has them. What was on my car was a 1 1/16" and it took excessive foot pressure even on moderate stops. I bench bled, installed, then pressure bled the system. I took it for a drive and the pedal does travel a bit more. But it takes way less foot pressure to stop the car. And it can lock up the tires where as before it could not.
It bolted right in and hooked right up to my reproduction brake lines without issue. Notice how the rear is curved and the front is square. Opposite of how the stock '69 ones were.