Up until this evening I was familiar with thumb-hole stocks only for rifles and trap guns; however, a quick Google search led me to the Website for Boyds' Gunstock Industries, Inc. at
www.boydboys.com. They picture a pump-gun with a thumb-hole stock (their Sterling model) attached. Unfortunately the Website is under construction and I was unable to obtain any further information.
Check this site later, or call them, and see if they have what you are looking for.
Good luck, and good hunting!