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The Tektronix DC503 frequency counter had an internal slide switch that you could set to provide a display in RPM vs. Hz. Never did see any other counter that did that. I though it could be a really handy feature but never did use it myself.
function generator in pulse mode, and an LED (high brightness white ones work best). frequency in Hz*60=rpm. as was mentioned above, a black mark or a sticker makes for a good "indicator" to use with the strobed LED