It's between .51 and .52. I checked some of my other Sharps and they are all ~.52. I saw the sporting Sharps (101 &102) in the T&T book and they are .38 and .44. Mine appears to have a much longer nose (from the top of the upper ring) due to the lack of the third ring. I compared this one I just found to my other Sharps and the diameter and length are virtually the same. The second ring is in the same position as the second (middle) ring on the all the regular three ringers.
I looked closely and do not see any trace of the third ring being there or carved away. Perhaps it is an anomaly and did not get cast/molded with the third ring?
