This is a Monroe Model G mechanical calculator that I restored. This model was made between 1918 and 1920, and is the bigger, heavier, much older brother to my Monroe LN-160x.
To add two numbers, enter the first number in the keyboard and then turn the large crank on the side one full turn clockwise until it clicks at the top stop. Then enter the second number and turn the crank again one full turn. It automatically carries. Here I'm adding 31337 + 5.
To subtract, enter the minuend in the keyboard and crank clockwise as with addition to enter it in the register, then enter the subtrahend and turn the handle anti-clockwise over a full turn until it clicks at the *2nd* stop. Here I perform 31342 - 50.
Yes, I said the 2nd stop. This older Monroe has some quirks they fixed in later models, like precise stops at around 1:00 for addition and 7:00 for subtraction. It locks the carriage between those stops so if you overshoot too far you won't be able clear the bottom register.
To multiply, enter the multiplicand, set the repeat key so it doesn't clear, then crank clockwise repeatedly for the ones position. Then use the handle in the front to move the carriage to the right and repeat until the multiplier is in the counting register. Here's 768 x 1024.
To divide, enter dividend into register, clear counter and keyboard, enter divisor, shift so divisor is over column it's less than. Subtract until you hear underflow bell, add to hear the bell again. Shift left. Repeat. Quotient is in top register, remainder below. Here's 145/12.
I can see why when this machine came out it seemed advanced compared to a Comptometer, but the quirks make it tricky to use. I had to record multiple videos because I would crank past the stop and lock the carriage. Later models fixed these quirks and are much nicer to use.
Clearing this machine is also quirky, you can clear the counting register with a clockwise turn of the top crank, but to clear the main register you must lift the carriage up manually before you turn the top crank anti-clockwise. Later models automatically lifted the carriage.