just a mechanical engineer'stacho
The extensions have various "feet" suited to the measuring point. For example have you seen the "countersunk" conical hole tapped into the hub of a high speed flywheel? Insert the leg with the rubber conical foot even while it is rotating and it will be "kept" and not wrench your arm off. Can you only see a flywheel rim? There will be a "foot" that is just a plain hard rubber disk. That's all it is... a measurer of rotational speed be it pulley, flywheel or whatever but it has a sensible speed range to stop you poking it where I wouldn't be putting my fingers. For fun look up a tentelometer for measuring the tension in a belt at one end of the scale down to a streaming tape drive at another. All nice mechanical devices but general purpose and not exclusively aeronautical. Still in use.