Hour meters on the older IH and virtually every other brand of tractor was driven off some component that rotated when the engine ran, like timing gears, distributor, injection pump...I've even seen tach's driven off the back of generator's & alternators. And above or below the hour display there would be a note "Engine Hours at such&such RPM" On Farmall M's, 400's & 450's which full load RPM was 1450 rpm it would state 1450 rpm...I have a tach/hour meter from a 350 Farmall on My Super H so it reads 1750 rpm. Years ago I remember the big V-6 GMC semi-truck Dad was driving had an hour meter in the tach and it said "Operating hour = 100,000 crankshaft revolutions". Which was close for an engine that was govered to around 2600 rpm but normally ran at less speed....100K divided by 60 seconds is 1666-2/3 rev./minute.
So any engine that runs the tach with a cable similar to a speedometer cable will record operating hours in spite of engine speed....for instance if the rated speed is 2400 rpm and the engine runs @ 1200 rpm it will take two hours to record an hour of operation.