I have searched my "Blue Book of Gun Values"' and cannot find the crossover match for who made your gun from Sears. The closest thing I could find was a Model 103.720, and Model 103.740 which were made for Sears by Marlin as their Model 59. When I went into the Marlin section, I could find no Model 59 listed under bolt action shotguns. Maybe someone else on here can shed additional light.
If you can get the gun positively identified, Numrich Arms Corp. carries extensive parts for many guns. You could do a search on their website for what you need.
Sorry I couldn't be of more help...........
