From some research and some conversations with a local hotrodder I thi k the early cars only had the dual brake "safety valve" with the plunger that slides to redirect pressure to the opposite system, ie front or rear in the case of a catestrophic failure in one of the systems and to as well indicate the failure with the brake indicator light.
A true "propoptioning valve" modulates the pressure to the rear wheels and usually has some sort of user adjustable screw or k ob to adjust the pressure to the rear wheels.
Most later cars and gm cars seem to have them integrated into the safety valve and have the adjuster screw.
There are aftermarket "proportioning valves" that cost between $15 and 50 that can be installed into rear brake line to adjust the pressure applied to the rear break system. there is a knob that allows for the adjustment to balance the pressure.
Do a google search for "universal adjustable proportioning valve"
As I mentioned above, I plan to do a 4 wheel brake job in january and have an aftermarket valve installed.
Ill post with the results.