This "+" mark has been found on the left upper side of the receiver on some "Second Model" Utica guns. It is near where the barrel breech fits to the frame. I am away from home and cannot post a picture. It is on page 78 of my newest book. A reader contacted me after finding this mark on a newly purchased toy of his. He is interested as to what it may mean. As I mentioned earlier, I am away from home, but I think I only have one gun with this mark.
Years ago when Mr. Callahan, a retired Savage employee, was providing historical information on these guns, I asked him if he would decode all the various symbols found on them, but he declined to do so. We may never know.