Hi Rocky,
Your granddad's external hammer shotgun may be a new but it is still has no manufacturer's marks to our knowledge. Bruce is probably correct at a 99 and 44/100% chance!
Without markings no one could tell you who made it. Sorry.
If you want to have it repaired and give to the please do so.
If you like, go to the Double Gun Shop forum and ask the members.
http://doublegunshop.com/forums/ubbthreads.php
Let us know if anyone solves this mystery.