A 1959 Browning Superposed factory two barrel set is one of the most desirable shotguns in the world. You could order one today for about twenty four thousand bucks for the basic gun and add about ten thousand for the extra barrel set. No kidding. And it wouldn't be as well made, and you'd wait for months, if not over a year.
Those forend and tang screws were custom made for your gun and the screws "indexed" (lined up) and engraved and slotted by the factory and you can't do that at home Grandson!.
The one and only place to have these done is at Art's Gun Shop. Art worked for the Browning service center in Arnold Missouri for about thirty years and Art can make your Grandpa's Superposed look exactly as new. At the same time he'll check out your shotgun and see what, if anything, else it needs.
Your shotgun set is probably worth between a bottom dollar minimum of a thousand dollars to perhaps three thousand dollars, depending on condition, originality, and options. But it's a family heirloom, and something else you need to consider. I've seen dozens of Superposed shotguns, I own a couple, and I'm a regular shotgun addict. I have never seen and picked up a Superposed with a a factory extra barrel set. I have seen them advertised, but I've never actually held one in my hands. They are not at all common. And, a late 1950's Superposed is the absolute high water mark of quality for a Superposed,,,and there are differences in build quality over the years.
Please don't have some jackleg gunsmith replace your missing screws. Don't even think about doing it yourself. You can't even buy a common, cheap new shotgun for what Art will charge you to make your Superposed two barrel set into a gorgeous gun.
Do not ever shoot steel shot through your gun, no matter what anyone tells you. Pick up a cheap late model used shotgun to shoot steel shot through. Expensive hand made shotguns like the Superposed are NOT set up to be pounded and battered by ridiculously heavy loads or steel shot or buckshot or slugs. A Superposed will fire hundreds of thousands of regular lead shot target or field loads and last for generation after generation if properly cared for.
Your Grandfather was a classy, classy guy.
Here's Art's web page. You can surely pony up to have Art put three screws in your gun,,and no whining!!! After all, Grandpa had to buy the thing in the first place.http://www.artsgunshop.com/