????? What this called

That is NOT a proportioning valve. It's a combination distribution block and brake warning switch, see the electrical terminal? It has a piston inside, and if front or rear pressure drops off dramatically compared to the "opposite end" the differential in pressure forces the piston to one end, activating the switch, and turns on your warning lamp on the dash.

It is NOT necessarily just for a drum brake car. Some had separate metering/ proportioning valves.