Talking about abacus ? I bet the third from the bottom on this page :
http://www.vintagecalculators.com/ht...lculators.html

Kingson Four-Rule

Martin

Joe Bramblett, KD5NRH wrote:
Surely somebody remembers these pocket mechanical add/subtract calculators.
I was thinking of trying to make a large one from wood as a math teaching
aid. Has anybody here done it? The mechanism is pretty simple, and it
looks like it would be easy to build, but we all know how the easiest
looking projects can turn into a mess when it comes to actual execution.