Star Adding Machine

The keyboard is comprised of "paddle wheels". Stick your finger between the two paddles that bracket one of the large numerals, press down until your finger hits the silver plate, and let go - sort of like dialing a rotary phone. The wheel springs back to its original position and the number you chose is accumulated into the total at the position of the wheel you used. If you're dextrous, you can rotate multiple wheels at the same time for speed. To subtract, use the positions of the small numerals (i.e. add the 9's complement of your subtrahend) and finish up by adding 1. To prevent subtraction from causing an undesired carry, press the red button at top corresponding to the most significant digit of your subtrahend. Clear the machine with the red lever on the right side.

And, it's portable! Flip the locking cover down, and raise the silver handle. Off you go.

The mechanics are relatively simple, sturdy and I think very reliable. The all metal construction can probably take a lot of abuse. It seems an eminently usable machine to me. This particular unit was carried by a milkman in his delivery truck according to the seller.

It was really dirty when I bought it. The cover had come unriveted from one of its hinged supports, and one of the wheels didn't spring back. I took it apart, cleaned and lubed everything, replaced a missing spring, soldered the support to the cover and now it works fine. But the lock is jammed and that's good because there is no key.