The DIGI-COMP 1 is a functioning, mechanical, digital computer that was sold in kit form. It was originally manufactured from polystyrene parts by E.S.R., Inc. starting in 1963 and sold as an educational toy for US$4.99.
The DIGI-COMP I contains three mechanical flip-flops that can be interconnected using thin vertical wires that are either pushed or blocked from moving by a number of cylindrical plastic tubes. The computer was 'clocked' by moving the "CLOCK" lever on the right side back and forth. Different configurations of the plastic tubes caused the Digi-Comp to compute different Boolean logic functions. The three binary digit readout of the state of the flip-flops showed the state of the binary logic and could be used to demonstrate operations such as addition and subtraction.
We even have the manual and the accessories to reprogram the DIGI-COMP 1.
You can see one counting to 7 in binary here:
Click on the image for a larger view.