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Victor Adder - Figure-Eight Calculator Back 
Victor Adder, A. A. Weeks MFG. Co. New York, circa 1905 The Webb Adder, or figure-eight adder, was patented in 1889 and was licensed or imitated by a number of brands such as Victor. The small window where the discs meet shows the total. The large disc is numbered from 00 to 99, and the small disc is numbered from 0 to 49, so together they can tally as high as 4999. One adds to the total by inserting the stylus into one of the holes on the larger disc corresponding to the number desired, and rotating the disk clockwise toward a stop at the zero location. When the large disc advances from 99 to 0, the smaller disc advances by 1 digit automatically. 
Here's a great collector's site with a good photo and instruction sheet for this adder: and here's a peak at the instruction sheet from the jmgoldman site:
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