This was the original rough model that Dorr Eugene Felt (1862 - 1930) created over the yearend holidays in 1884-85. It was a remarkable implementation of a conjoured design that simply "all came together" at a certain critical point.

The Comptometer was the first practical key-driven calculator with sufficient speed and reliablility to bring significant economic benefits to the processing of business data.

Prior key-driven design efforts had failed to solve the twin problems of over rotation (tendency of a dial to spin past its intended stop) and carry (incrementation of the next higher order on overflow). Felt's design resolved both while adding high speed operation.
Comptometers were available in 8, 10, 12 and even 16-column versions as well as for British money (sterling), fractions, etc on special order. Except for the wooden-cased, A and B models, serial numbers appear next to the "1" key of the leftmost column of keys.