I discovered one of these cute little instruments at a hamfest, and I remember that these were discussed here in RCM before, but I can't find any posts with google groups search. I've included a proper subject line so that it can be searched.
This tool is in an original box (and 2 contact tips), but doesn't contain the instructions. What an odd looking speed indicator.. the input shaft is a worm that directly turns/drives the numbered dial. There is a spring-disk on one side with a detent bip that contacts a high spot on the dial at only one place. This would effectively produce a click for each rotation.
I confess that I really don't know how this tool indicates speed.. and I'm looking for an explanation from someone wiser than myself