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