Might be worth pulling the stock off to see if it is signed inside. Many stockers sign their work with a stamp inside the barrel channel where it isn't visible to casual viewer.
www.doublegunshop.com/BBS has a forum dedicated to custom single barrel rifles, and many guys there really dig--and know--custom makers, especially those that used Springfields. If you can find a stocker's stamp and can't interpret it, I bet some of those guys could.
Not my style of stocking, but very impressive workmanship! Stocker definitely was an expert.
