I recently acquired a Buck 6-jaw chuck from a grind shop auction. The backing plate had a radially adjustable intermediate plate mounted to it, intermediate plate having the chuck register and then the set-tru chuck (guessing this setup might have allowed for turning or grinding an eccentricity?). The backing plate has 1.5 x 6 tpi threads and has a collar. The collar dia is about 4.182 at max dia (quick caliper check, about where the red Sharpie line is on the 2nd image) but the collar is barrel shaped. I have a second one of these backing plates without the faceplate feature (also has the same barrel shaped collar). There were a couple of 2nd opp lathes (possibly Hardinge, now really regret not paying any attention) and numerous grinders. The chuck does not look like it came off of a grinder - no visible grinding dust. Any ideas as to what the backing plate might fit? The backing plates are not of any use to me and I would like to turn them into a bit of tooling money if possible. TIA.