Master Cylinder

Well that does shed some light on my question. The MC's are cheap enough that when the time comes to do the conversion I will by the proper MC. Since I currently have drum/drum, I'll get the MC for that application. I was simply thinking ahead for the build. I sure would hate to have faulty brakes with the wrong MC for the set-up now.
The correct MC is actually pretty expensive at the moment. Regardless of the photo, most production rebuilders have been simply substituting using drum/drum MC's with no residual valve in the rear outlet for the past 20 years. :( It will work OK, but requires more frequent checking of the fluid level.

Good reading on the system here: 1967 Chrysler Hydraulic Brake Master Tech - Session 232

and here; 1967 Chrysler Disc Brake System Master Tech- Session 233