That is Simpson's import mark. They have a fairly active import service and import (re-import?) a large number of U.S.-manufactured guns, many from a single supplier. I've purchased a couple of shooter-grade Browning Medalists from them that were imported by them and they are marked the same. They were marked on the bottom of the magazine well, making it unobtrusive. I've also not purchased an otherwise pristine one-liner K22 because it had their import mark on the left side of the frame, right below the cylinder window.