It isn't too often that I am blown away by a tool, but this one is something. No name, numbers, pat's, tells me it was made in a shop or a school by an accomplished student. To prove his worth on a mill, shaper, lathe. Could even have been forged to shape then finished on the rotating tools. Have never seen an adjustable opening that was round. Was finished, inspected, passed around the journeymen and masters of the shop, then taken home and put away. Othewise it would be full of nicks and dents. I could be wrong, but this is what it seems to be to me.