Cams with the shaped required to draw the digit for each digit arm. You can see there's a rod with various cams on for each digit, four of them in total. I imag

Cams with the shaped required to draw the digit for each digit arm. You can see there's a rod with various cams on for each digit, four of them in total. I imagine there's some sort of slider to engage a cam for the "current digit" so when the redraw triggers the selected cam is the one used for the motion. Many automata use a similar design to to achieve preprogrammed movements, like writing, drawing or playing an instrument.