Before I obtained my more sophisticated measuring tools this single-use tool was made specifically to measure the width of a ridge machined deep inside a bore in a part I was repairing. Its use is straightforward. You open the two disks at the end of the tool by compressing the spring, allow the disks to close on the ridge to be measured, tighten the thumbscrew to retain the measurement then withdraw and measure with inside calipers.
After its moment of stardom it went into a toolbox drawer where it lived, unloved, for many years. Then, one day, I needed to place a nut on a bolt deep inside my latest mad genius creation and it hit me, use that "thing" you made so long ago to hold the nut.
As a nut holder, it could be improved. Slightly larger disks with some roughening to better grab the nut would be a boon.
Probably most folks will have no use for such a tool. I offer it more as an idea that you might adopt to your particular needs, just as I did.