The Victor Adding Machine Company (as of 1961, the Victor-Comptometer Corporation) has been making mechanical calculators since 1918, both in ten-key format, as you see here, and full key with up to 11 columns.

The little indicator above the keys tells you how many digits you've punched in.  Pulling the crank on the right causes the number to print out on the roll of paper.  To total the numbers you've entered (and reset the machine), you hold down the total button on the upper right of the machine while cranking.  If you make an error with an individual entry, just reset it with the slide directly to the left of the keyboard

The switch on the upper left is a "repeat" switch that is used for multiplication.  Just input a number, depress the switch, and crank the lever.  It will print the same number as many times as you crank.  For instance, if you want to multiply 13 x 5, you'd enter 13, depress the switch, and crank five times.

If you want to multiply 13 x 65, you don't have to crank 65 times!  Instead, crank five times, and then push the "zero" key — this shifts the register over one digit so you're not adding 13s but 130s!  From there, you crank six times and voila!  13 x 65 = 845.

Not only is it functional, but it's pretty.  Perhaps not quite so striking now that it's made of gray metal rather than black Bakelite, but the art deco lines of the late '30s original are preserved.