This is an accessory for the Acorn BBC Home Microcomputer which connects to the home computer through the 1MHz bus. It has its own self contained power supply and plugs into the 240 Volt mains. The "Music 500" is a synthesiser which can be programmed to play background music (I think)' It may require a driver program on a sideways EPROM..

The case is missing.

The photos are part of the description.