Herb Chambers: Saw your post & since I've got a 12 Ga 1964 vintage Superposed Lightning (not trap model). I thought I'd make a few comments. I use mine for trap (it's M&F) and for those late (pheasant) season windy days, when those "ole roosters" are out to 40-45 yards before that 1 1/4oz load of copper plated #5 catches up w/ them.
I'm sure you're aware that a high percentage of superposed guns produced from late 1966 to early 1972, are prime
candidates for possibly being stocked w/ what is commonly referred to as "a salt wood stock".
The potential resale value of your gun obviously will depend on many factors: 1/ salt wood stock or not; 2/ if it is(was) a "salt wood gun" has Browning replaced the stock?; 3/ Is it long tang or short tang; does it have a Round Knob or Flat Knob? 4/ condition (%) of blueing remaining; 5/ condition of stock & forend; 6/ tightness of action; 7/ is gun all-original, etc. etc etc.
Since you've obviously used the gun for quite some time (my supposition), why don't you explore what Briley (or someone similiar) would charge to install change-out choke tubes and re-configure the stock to fit you for clays? That way you'd continue to use a gun that you are extremely familiar with & not have the hassel of selling or trading or whatever.
Just my thoughts, for whatever they're worth. "Grizzly"