It is an assembly of
two changeover valves.
A changeover valve contains a free-moving piston which moves according to the higher incoming pressure and delivers air to the central (outlet) port - which can become the inlet to a second COV (change-over valve).
They are arranged to operate so that a system takes air from whichever circuit is pressurised and can alternate footbrake and handbrake air - selecting the higher pressure without mixing the inlets or can use supply air from tanks or compressor to feed downstream circuits.
These COVs can incorporate pressure switches (see the flange on the upper COV) such as stoplamp illumination or low-pressure warning (yours don't).
Tracing the airflow of your illustration, air pressure would flow from either of the top pipes (straight or elbow) - depending on which is the higher pressure - to the lower COV.
The outlet of the lower COV (the middle horizontal straight pipe) would receive the higher pressure between the upper COV outlet and the bottom horizontal pipe with the elbow inlet.
Seemples . . .
