I think he means one of these!
Mine was more like Fatters', but I think it had only four digits and they were in line horizontally. It may have been mounted on the
left fork, so that the 'units' was nearest the toothed wheel, this would have made sense as the escapement mechanism would be more simple in design.
Another reminiscence. Not exactly a bike bit, but nevertheless a useful accessory, I remember my father giving me, in my teens, a map-measurer. No photo I'm afraid, it looked like a compass with a dial and needle calibrated in miles, driven by gearing from a small steel wheel (about 2-3mm) projecting from one end of the gadget. Calibrated for one-inch-to-the-mile maps (1:63360, now obsolete!), before the days of Google Maps and suchlike, it was the only way of measuring out a ride in advance. It was not very accurate!
Has anyone got a piccy of such a thing? I think the face opposite the mileage display
was a compass...
[edit]
good old google has found me an image of something just like it! So it seems they still exist.