So I did more work on this today. I called remington and they explained a couple of things. First of all that there are two recievers, a large and small version. Second the difference between the magnum and the standard is one gas port for the magnum and two gas ports for the standard. So, a pre-1977 standard 20 gauge barrel will fit on a pre-1977 magnum 20 gauge frame.
They gave me a couple of contacts and one of them had just gotten in a 26" 20 gauge barrel with a vented rib and improved cylinder, new in the box. So for $352 including shipping I am now the proud owner. Man, I have other shotguns that cost as much as that barrel. Oh well.
Firearms stand next in importance to the Constitution itself. They are the American people's liberty teeth and keystone under independence. The very atmosphere of firearms everywhere restrains evil interference. - George Washington
Peace is that brief glorious moment in history when everybody stands around reloading.' -Unknown