For measuring cylinder bores. Mostly for checking wear, which shows up as taper and out-of-round. The setting tool is clever, being very simple, but accurate, although it is time-consuming to set up.
I have one that I picked up 30+ years ago.
I finally got out to hunt at the Steam-Up yesterday. Lots of things left behind, but the regulars overlooked a couple of useful things:
A 4-place rack for parts boxes, with three Bowman and one Kar box, all empty. Lots of storage for $30.00!
Bonney RG-63 tubing wrench with Loc-Rite...
I bought one for the same price a number of years ago. It had the plug cut off the cord, and the seller thought it was to keep a couple of teenage boys from trying to put it to use! Safety first. I tested it for shorts and grounding, then put a new plug on the cord and it works perfectly.