Whatever it is, it looks dangerous! - it would need to be opened to get any idea about what it might be.. but it is not a Theremin!
Looking at the back there is a jack socket (and something which MIGHT be another, possibly RF connection point).
My guess was first inclined towards some form of active antenna - perhaps a broadband RF field strength meter, perhaps including a rectifier to give a DC field strength output on the jack socket.. or perhaps its a part from a scapped Dalek..
To be honest, it could be almost anything but it is not a Theremin! .. It is likely to be some sort of RF reciever, but has no tuning control, so is either broadband or tuned to one specific frequency.. It says "gain" which would imply amplification - and it has power input, so is likely to have an active amplifier..
Is this yours? - If so, open it up and photograph its innards - and DONT plug it into mains.. that crappy ungrounded lead into a metal box looks potentially lethal! ... If you dont own it, I suggest you forget it - its not going to be anything exciting or useful, of that I am sure!
