It is a marker for measuring out lengths on a curve. I made one years ago for use in a steel shop.
Smarty pants!
It’s called a traveler, used by a wheelwright or blacksmith, to measure the length of a metal tire that would go on a wagon wheel.
I have one that has a manufacturer/patent mark; and 2 others that are hand forged and more primitive.
I have THREE of those!!
Well; not THOSE those; but wheels that measure.
A very small one that you can adjust the distance for the correct scale on a map; and then you can roll it along the road, river or trail and see the distance displayed on an LCD window.
A second one for use in a building. A mechanical odometer type counter and a 3” diameter wheel.
And a larger one for outdoor use by rolling it along the ground.