This is a device that goes hand-in-hand with the slide rule. Just as the slide rule replaces the calculator's ability to do higher operations without electronics, the addiator is capable of rapid, painless addition and subtraction without needing pen & paper.
They haven't been produced since 1982, but can still be found on eBay and in a pinch can be fabricated with simple tools.
Here's the parts:
When zeroed, the windows show all zeroes. Our cheap version has no reset mechanism, but the commercial ones are reset with a little bail handle on the top.
Basically all you have to do is enter your digits by holes from the desired number to the bottom. When an overrange condition appears in a window (shown by a red arrow), that digit is carried by a simple movement of the stylus; dragging the lowest hole to the top (thus subtracting 10), following across to the left and down (thus adding 1 to the next column).
I put together an example using JPEGs, but I found a good example of it's use in this video: YouTube
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