There's some good summaries of the valves in the master tech series.
However the level of detail for the visual cues oldmanmopar posted above is superior.
One possible source of confusion is terminology:
Distribution block in Chrysler books is generally called the safety switch. (Because after '66 it includes the brake failure switch)
The hold off valve is commonly used by people here when refering to the proportioning valve.
Many mechanics and IIRC some Chrysler materials use hold off valve for the metering valve. Metering valves are for the front brakes and were not on any factory A-body AFAIK.
In Chrysler master tech books, fixed calipers includes the 4-piston Keley Hayes for a-bodies.
