See the
picture of it for best results. This one's on my stuff for sale site because it wasn't picture gallery quality. :-)
It's a 2MB RAM expansion device for A1000. It goes in the side expansion port and has a pass through so you can jam lots of other expansion devices on your A1000 until the cows come home or until you put on a device with no pass through. (Or until you burn out your power supply or other some such pesky reason of reality.)
I'm "for sale"-ing this totally working awesomeness as soon as someone tells me how much it's really worth. Otherwise I'd do that "it'll cost ya $1000" gag that nobody seems to even get. :-/
If you go to the
Amiga Hardware Database, the pictures of the main board and "upper deck" are there. They are of this exact device. I submitted those pics a few years ago, myself.