This is essentially a chain of flip-flops forming a binary counter fed from either a multivibrator or a telephone dial.
Numbers entered through the telephone dial can be added or subtracted.
Here the binary counter is fed from the multivibrator at around 64Hz.
Here the numbers 1 to 9 are entered via the telephone dial and added to form the binary total 101101 shown in the top lights (45 in decimal).