Roland MC-500 micro composer.
This is a four-channel (!) MIDI sequencer with one MIDI in and two MIDI outs. People laugh when they see it but it was the bees knees in 1986 (Go West had two and so did Michael Jackson) and I've been using it so long that I can really work fast on it. My fingers go all blurry when I am quantising etc. It works, the timing is really solid, better than a computer, the small screen is crap but I like it. I bought it out of Loot in the early 90s and went round the guy's house in New Cross to pick it up, we ended up making a piano-rave track together which I've still got somewhere on tape. You don't get that with eBay!
MC-500 - what were you expecting?
MC-500 at Sonic State here
MC-500 at Harmony Central here
Roland manual downloads here