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.
![](/community/proxy.php?image=http%3A%2F%2Fi1368.photobucket.com%2Falbums%2Fag200%2FDan51%2FMisc%2520VAF%2520Illustrations%2FIMG_20160110_102513900_HDR_zps9lbpaqjh.jpg&hash=efabfc59551a3241b912cdb0b772a077)
So what is its purpose? When would it be used, and how/why? I don't know.
![](/community/proxy.php?image=http%3A%2F%2Fi1368.photobucket.com%2Falbums%2Fag200%2FDan51%2FMisc%2520VAF%2520Illustrations%2FIMG_20160110_102513900_HDR_zps9lbpaqjh.jpg&hash=efabfc59551a3241b912cdb0b772a077)