According to this page it's a hub micrometer…..
**LINK**
"Dedicated to measuring the length and thickness of steps in a pipe."
…but I think it's designed for the frame to fit down a bore and allow the measurement of the width of a hub.
Edited By Jon Gibbs on 28/03/2018 16:14:08
Edited By Jon Gibbs on 28/03/2018 16:17:08