First, here's the cute answer: If it's "Steven's" shotgun, you had better return it to him.
Obviously, it's a Stevens gun, probably made by Savage Arms.
It looks to be in pretty good condition.
Somewhere on it, perhaps underneath the barrels' breeches, it should tell you its gauge and the barrels' chokes. Make sure, before you try to buy ammunition for it.
In the same place might be notations about its model number, and maybe even a date.
If you have any use for a shotgun, I suggest keeping it. After all, it's a family heirloom. Besides, a DB shotgun is useful as a home defense.
Further, this shotgun is no longer made, and may have collectors' value. Look at the GunBroker site to find a price, or look in the current edition of the Standard Catalog of Firearms. Stevens double-barrel shotguns are presently worth from $400.00 through $1,400.00, depending upon model and condition. According to your photos and those in the references, the price range of your particular gun might be closer to $800.00 through $1,000.00, depending upon its model number.