To close out this thread, here is correspondence between Felt himself (the inventor of the Comptometer) and Zenith Pub. Co.
Subtraction was weird. You'd subtract w/ addition by the complements method, but would first subtract one from the number, then press down that number using the small digits on each key. You also slide the correct metal tab at the bottom to stop the carry. Here's 9 - 2.
You can perform many calculations surprisingly quickly and w/ one-hand. A well-trained Comptometer user was probably as fast as you'd get until electromechanical calculators. Here I'm calculating 1024 x 768, which just requires pressing 768 a few times and shifting left.
The Comptometer had a superior design. Pressing a key immediately updated the register, no need to pull a lever (the lever here clears the register). This design was so effective it stayed essentially the same (with minor improvements on error prevention) through the `50s.
This is the carry mechanism for the Monroe. The bars form a wave and the last few move, but were completely seized up. A few hours with some oil, pliers, a screwdriver, and ultimately a heat gun, and it finally loosened up. Subtraction finally underflows all the way to the left!
Of course if you can add repeatedly you can multiply, so this machine has a "repeat" button in the bottom right corner. Once pressed, any number you enter stays pressed down after you pull the addition lever, so you can add over and over just by pulling the lever.
The Burroughs is a true adding machine--it can only *add* numbers. Notice the limited buttons on the keyboard. If you ever heard someone refer to a modern calculator as an "adding machine" it derives from the fact that originally that's all they could do back in 1908.
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