The Comptometer to the right is a Model F, made between 1919 and 1920. It is the mass-produced successor to the smaller (and rarer) Model E (1913-1914) to the left. The Model E introduced a "control-key" mechanism to prevent errors from half-presses, but Model F simplified it.

Like with other Comptometers, you just press corresponding keys to add. Trained Comptometer operators performed calculations by feel (odd keys were concave, even were flat) and mostly one-handed so their eyes and left hand could stay on the sheet of figures. Here's 31337 + 5.

To subtract, use the small digits on the keys instead of the large, subtract one from the subtrahend, and hold down the correct switch in the front to prevent the one from carrying. To do 31342 - 42, I press 41 in small digits (58 in large digits) while holding the front switch.

Multiplication is easy and fast. Just do repeated addition for the first digit in the multiplier and shift left until each digit is accounted for. Here is 768 x 1024.