It seems to be an expansion device that lets you plug in a 1-button joystick. In fact, when I found it it had an Atari 2600 controller hardwired to it. In the meantime I've replaced the joystick with a joystick connector, but I'd really like to know what kind of machine this thing is supposed to plug into.
The connector is 50 pins, has no keying, and is female. Logic dictates that this is probably designed for a machine that ordinarily has no game controller ports, but the only one I can think of, the Spectrum, has a different expansion port connector. So it has to be for some other machine. A Google search of both newsgroups and the rest of the internet reveals nothing.