That's not impossible for a simple electromechanical calculator either. An attachment similar to those paper drums seismographs use would suffice while still be

That's not impossible for a simple electromechanical calculator either. An attachment similar to those paper drums seismographs use would suffice while still being simple enough to be trustworthy; a video recording of the counter along with maybe an array of debug LEDs would suffice too now that I think about it.