You may well have a very unusual "factory" stock, or it could be a replacement as you say. Ithaca made some wierd and wonderful stuff for their customers, such as Lon B. Smith's trap guns --



or the original Chas. Askins Magnum-Ten --



It would be great if you post some pictures of your gun. If you provide the complete serial number perhaps Walter Snyder can provide some information on the gun.