It looks like what happens when you set a damp glass on a wood table. The white ring is in the finish, called a water stain . Was the stock ever exposed to moisture, it looks like it may have been standing up and the butt area got damp . I have heard varous home remedies (do google search) which involes mayo and ashes , vinegar and olive oil and other such fixes .
None have ever worked for me....you could always see the mark.
I ended up refinishing the stock or table top .
Find a method , try it and if the results aren't acceptable, a stock refinish is not hard.
If it is on the surface of the finish , linseed oil and 0000 stell wool lightly rubbed on the surface might remove it.
Gary