It's...

...A knob from an EMT140. Well technically it's a handwheel - that controls the asbestos shutters that control the reverb time. We refurbished two of them around fifteen years ago that alleged to come from Decca Studios. They were in fairly poor condition, so it was a bit of a punt but we got one really good unit out of the parts. This is the handwheel from the unit we couldn't salvage. Serial number 743