Try this marinade for ducks and geese- I filet out the breasts on Canadas and bigger ducks- after a soak in cold salted water-to draw any residual blood- marinate 48 hours in Buttermilk, then pat dry, pepper to taste and grill with an olive oil and powdered grilling seasoning of your choice, when a sharp knife into the thickest part just leaves a trace of clinging blood, they are done.
What is the black barrel cover on your fine double gun. Is this a forearm extension? I am guessing your gun was made prior to the Beavertail style of forearm was used on some side-by-side guns.