I built this calculator out of K'NEX on 7/14/2005, with the center column made the day before. It uses octal (base 8) because the white K'NEX connectors have 8 connection points. It does not automatically handle carries & borrows. It uses a stepped drum to add & subtract. Addition is accomplished by making one full turn of the crank (the yellow lever at the top). Subtraction is done by turning the crank in the opposite direction as for addition. Which direction does which is arbitrary & therefore user-defined, although clockwise seems to be the natural choice.
In the pictures, the digit-entry gears are all in the 0 position (at the bottom), as are the value gears (pointing toward the front), & the crank is also in its rest position. There is actually nothing to distinguish which side is which, but I have arbitrarily pointed all the pointers in the same direction & called it the front. An additional piece could be added to one side to make the distinction more explicit.
Top view
Left side view
Front view
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