This is a Tasco Pocket Arithometer. It was probably made in the 1940s but is based off of older Arithometer designs. It is sized to fit easily into a shirt or coat pocket. To add numbers you place a stylus in the appropriate row and drag down, which adds to the existing total.
As you slide rows down you notice some numbers in a row are colored red. That signals that when you add those numbers, instead of sliding down, you slide up and around the "shepherd's hook" to carry the one. Here I added 5.
To subtract, you slide a panel from the bottom upwards, which aligns new numbers with the holes. Now you slide up instead of down, unless you need to carry a one, in which case you slide down around the hook. I'm not a fan of this part of the design--it is pretty cumbersome.
To zero things out, you pull on a lever at the top. Basic tallying, like scrabble scores, can be done pretty quickly on one of these after some practice. You can also do multiplication via repeated addition and left shifts like with other adding machines.
@kyle hey Kyle which one is your favorite? You have quite a few mechanic calculators there.
@Konqi Of the pocket calculators it would have to be the Addiator Duplex, which I will feature in a few weeks.
Between the desktop ones I own that's a tough call! I haven't featured all of them yet but probably would choose either the Comptometer or the Monroe LN-160x if I had to pick one to use for real work. The former for addition and multiplication, the latter if I also needed to do a lot of subtraction and division.
@Konqi That is a very similar model as mine, but with a few more rows of digits. Beautiful machines.
For adding machines of that era the Comptometer is faster for pure number crunching, but then with the Comptometer you couldn't print out your calculations on paper tape or a ledger like the Burroughs.
@kyle Yes, those are indeed beautiful machines, better yet, they last forever, the way devices of the past were manufactured is simply amazing.
@Konqi The Burroughs Class 1 wins for overall aesthetics with the beveled glass sides and it is the one I'd put on most prominent display inside a home.