This is the compressor from a series 2 discovery, it usually lives on the side of the chassis under the passenger side, there is a top cover missing. I saw this at Sodbury and thought about buying it.
As I recall
"A" is the intake.
"B" is the air out via a solenoid which has connections not shown in the picture.
"D" is I think a pressure regulator, the top bayonets off and has a spring underneath.
"C" is the connection to the solenoids marked "E" which is full pressure from the compressor, these solenoid valves control the rear air bag pressures.
"F" is obviously the plug, try live into the brown wire.
This is all from memory when I played with one just the same last year, please don't blame me.
Very fine compressor and the solenoid valves make it quite versatile.