And if you have one of Clain's counters, the counts per revolution would almost certainly be exactly 20. The reason I save "almost certainly" is that sometimes there was a counter which did 26 counts. For those who are interested, the 20-count ones were for 20-inch wheels and the 26-count ones were for 26-inch wheels. In this way they could use the same mechanism on the handlebars for both size bikes.