The following shows how to add 1 to 99 to get 100. The calculator starts at 99 as shown below.
Then the stylus is inserted into the rightmost '1' position and slid down, which causes a red flag to appear in the rightmost result window, indicating the need for a carry operation.
To perform the tens carry, the stylus slides the rightmost ladder from the bottom to the top where it continues leftward over the 'shepherd's hook", which leaves a zero in the right most result window and causes the next column to advance by 1 (see below).
Now we have a red carry flag in the next column, so we must repeat the tens carry motion for this column also -- up and over the hook, which causes the next column to advance by 1 as shown below.
So 99+1=100, not bad for an inexpensive old calculator. Finally, to clear the result window, we can lift the clearing bar (as shown) to prepare for the next calculation. This calculator performs subtractions via the slidegates beneath the result window, and negative numbers, if necessary, appear at the bottom.