Mark, you said in your PM that this gun has a very high serial number. Previously the highest serial number Norwich guns I've recorded are in the 11xxx range and the earliest Woodstock guns in the 9xxx range, so a lot of overlap. In the 18xxx range we see guns that don't have the rectangular extension of the barrel lug through the bottom of the frame. Gun with the extension --





Late gun without the extension --





It is hard to tell from your three pictures if this gun has the extension or not.