I don't quite understand the ?

If you are inquireing about how they work,
you turn the reset knob and they will work as if the meter was turning the unit, the back hand will keep up with gallons as the outer hand turns, then with a reversal of rotation of the reset knob , the unit will reset. They will also chime on the gallon.

All the original clocks I have ever had, all you can do is reset them.....not much fun there.

Veeder Root Rebuilds.....since 1987
Veeder Root Identification CD
Gas Pump Clock Repair