Abacus

[jvrjenivs]

So how was the counting done on one of these?

It's quite easy. You start with the lane where the I is the sign. Every 'rivet' on the lower part has value 1, the upper one has value 5. So, start with all down, and put one up will make 1. Two up will make 2, etc. After 4 your lower rivets are all gone, so the upper should now take over, and all lower are back to zero. Now you have 5 and can go to 9.
For 10, go to the lane with the X and on the lower part every rivets is worth 10. Etc, etc.

Hope this makes it more clear.