The Mephisto Mobil is one of my favorite old chess computers. It is a neat little gadget that lets you use various Mephisto MM chess computer modules without having to carry a large board around. The Rebell 5.0 is also compatible with it. It is a chess computer from 1986 with 9 levels. It has an estimated rating of 1850ELO.
The Mephisto Mobil is designed specifically for testing these chess computers on the go. It has a few buttons that you can use to enter the moves. It also has a foldable magnetic chess computer and tiny flat pieces, so you can follow the games. The Mephisto REBELL 5.0 is pretty rare but you may still be able to find it on eBay.