Me thinks it looks like something to be attached to a long board or steel stud for scribing REALLY long radii. Say more than other gismos attached to a yardstick. Or for sliding along an irrigular permanent surface to say, get the bottom kick plate of wooden steps to fit perfect at the bottom and against the last wooden step. I had to "rube" something like this when restoring Mom's back wooden steps against the brick patio surface. Yep, it worked, trimmed the last kickplate with a jigsaw. Couldn't judge what was there first, the wood or the brick patio.