I recently rediscovered an old hand computer, which was in my desk drawer for ages: an Addifix-9.
The Addifix was sold in the 1960s by a no longer existing German mail-order company called Neckermann. (Well: actually the brand Neckermann still exists, but under a new owner.) A stylus is used to make additions on the one side, and subtractions to the other.
By the way: I really like the English name for this kind of tool: number cruncher… 🙂 (And no, it’s neither a slide rule nor an abacus, although similar.)
A video demonstrating the calculator is included in a newer post.