Quote Originally Posted by jonh172 View Post
That appears to be the final inspector mark when that rifle was re-built during WWII in USAicon.
It definitely follows the general format and feel of the final inspection mark after a rebuild. So much so, I'm also quite confident that's what it is.
The theory you found is interesting. Too bad we don't yet know for certain. The likelihood of that information ever surfacing is shrinking as time goes on.