He posted Meter and proportion, so that would be disc set up 70 up B & E though other models may have used it too. Meter holding off pressure to disc and proportion to limit pressure to rear drum. The distribution block/Dp brake warning is the 3rd and always present piece. Mopar did start to combine the valves into 2 and than1 piece later.

1st thing to establish is whether it is a proportioning valve or PRESSURE DIFFERENTIAL valve. The PD valve does nothing more than turn on the brake warning light if there should be a loss of pressure to 1 side of the system.
This is a common misconception that started many years back

Actually the master is where the dual braking comes from. Lots of vehicles built without the differential valve. The differential valve usually activates a light and some cut off flow to the leaking side, but not all.

1967

1sr engineer: Hey look, I designed a brake valve block so that when a lower pressure or leak occurs, this piston slides over and lights a light!

2d engineer: Hey good. But, when the piston slides over, why don't you have it cut off the port to the leaking side. That way you'd have a safety factor.

1st: I can't do that.

2d: Why not?

1st: Because 50+ years in the future, a guy on the internet is going to say that isn't how its supposed to work.

2d: What is the internet?

1st: I don't know, but I think in the future, those folks will be smarter than us.

If the distribution block sealed the low side to stop the leak, how would you bleed brakes? The Master training goes through this dual MC pretty well, the pistons do have tail piece to allow for direct contact to keep front and rear isolated in the case of a bad leak.

It takes a sharp pressure spike to set it over. Just take a look at your valve block, look at the front lines. You can SEE that the piston will cut off the ports just the way the lines are on there.

But, I give up, you guys have dual brakes no matter what. One question, when you bleed your brakes, did the pedal go to the floor? I'm outta here.

This is the definitive training answer to explain it. From the 67 Master training. Unfortunately, the slide show link doesn't work, but watch the film movie. MC covered first, distribution block around 11m 27 sec. It clearly shows how the port does not close. The barbell shaped warning light trigger does not move far enough to close the outlet ports because of the warning light stud stops it.

https://www.web.imperialclub.info/Repair/Lit/Films/232/

Stuff changes. From the 73 FSM, when talking about pressure bleeding the brakes it states the following.