Mystery Tool
 

Here's what I know. It's made by Starrett. It counts revolutions of the shaft (actual number, not RPM). The shaft has a rubber cap on the end assumed to be for friction. The rotating dial counter is shown zeroed. It will count in either direction, 100 revs per 360 degrees. The thumb-forefinger handle on the left is stationary.

So what is its purpose? When would it be used, and how/why? I don't know.

Just off the top of my head--------something to do with threads per inch, or something to determine gear ratios.

I am leaning toward gear ratios.

Lifter leakdown? Merwin Hulbert revolver suction meter?

Mystery tool
 

Speed Indicator , circa 1920-1930 ish .

I have one also, given to me by my father in law who ran a big lathe back in the days.

It?s a revolution counter, placed on the end of a rotating shaft for a specific time indicating RPM.
Was developed for machine shop use when machines were powered by line shafts.

Machinists RPM Guage

https://www.youtube.com/watch?v=4RLnwgp6YcM

Speed Indicator
 

It is a Speed indicator. It has several uses in machining. It can used to measure RPM when used with a watch, but mainly it is used for speed rates when cutting.
see: https://www.youtube.com/watch?v=SLWlkobzKgk

Tach
 

A practical use today would be to calibrate a variable speed Bridgeport J head.

https://www.youtube.com/watch?v=4RLnwgp6YcM

If you look at an old engine shaft you will see a conical depression machined into its end. The rubber cone on this tool fit into the depression. The fellows are correct...RPM could be measured with a watch.

Jim

Thank you gentlemen. I like the cutting speed measurement with the wheel!

It came from the toolbox of an old friend and mentor who passed recently. I have other ways to measure RPM, so I think I'll retire it to an honored place on the shop wall.