This is a pair of Addometers made by Reliable Typewriter & Adding Machine company. They were made from 1930-1960s. The top one is a Model A I think from the early 1930s due to "patent pending" text on the front. The bottom one is a Model C I think from the 1950s.

From a distance they look similar but the Model C was designed for adding Imperial measurements and has a dial that goes up to 11 for adding inches. Both models feature a dial for fractions on the far right, where Model Bs only have decimal dials. All models have a ruler on top.

My Model A came with the original case, which was designed with a notch cut in the side so you can access the stylus storage and the clearing lever without removing it from the case. It also came with the original instructions!

To add, put the stylus in the dial for the corresponding digit using the larger numbers, and turn clockwise until the dial stops. The mechanism automatically carries the one without having to move in different directions like with a "shepherd's hook" design. Here is 31337 + 4.

To subtract, use the smaller digits on the outside of the dial to enter the subtrahend and rotate anti-clockwise. The mechanism also will decrement from the left side automatically. Here I'm subtracting 50 from 31341.

To multiply, perform repeated addition and shift left for each term in the multiplier. Here is 12 x 12.