Although it has a superficial resemblance to one, that's not a tube socket. That's a five-pin DIN connector.
You often see that type of connector used on MIDI equipment, but I doubt this is generating a MIDI signal. Of course, that type of connector has been used for a variety of other things, too.
So, it's a bit of a mystery...
It really intrigues me as to what this instrument was plugged into.