It's...

...The Truvox Radio Jack. Long before the RF Nomad we had to make do with this. Inside is a nasty little AM radio that can tune to two stations via screw presets underneath. The switch flicks between the two presets and the interesting knurled knob is actually the binding post for the aerial. Clearly it begs to be plugged directly into a modular so I did. Oakley EFG Deluxe with the gain set to the max and I could just about pick up a taxi somewhere down the valley...