By the looks of what may be a Crest that has been removed from the top of the recv'r ring,,I suspect those remaining marks are from a dovetailed scope base installed at one time
then removed and the space welded up. Maybe the base left in place and welds run around the edges to fuse the metal and base in place.
The marks left behind form straight lines and when looking at the pics that show that area, they do (to me) show the outline of a dovetailed scope base.

Welded up and then faced off. the less than perfect job left those small void lines behind.
Sedgley used welding quite a lot in his workings.