more infor about this old side-by-side

[pbaron]

I have inherited my granddad's double barrel muzzel loader and wonder what info anyone might share about it's history, date of manufacture, etc.
It has twisted Damascus steel barrels, double triggers, is marked “COLLIS” above “OXFORD” in a double line oval on each hammer side plate and "COLLIS OXFORD" on the top rib. Under the forearm on each barrel it says "STUBBS TWIST", presumably referring to the barrel steel, and it has no sn. What little I could find on the net suggests Collis was a gun smith in Oxford, England, in the late 1700's. Any more complete or accurate information would be greatly appreciated.

[SDC]

Is this a percussion gun, a flintlock, or a flintlock converted to percussion? The percussion cap was invented around 1820. My books also say Collis was active around 1800, but no more info; you might be able to nail it down a little closer if you take the barrels off and post a picture of the proofs (there should be a cross-key in the forearm that you can tap out in one direction, then lift the barrels out of the stock, to get a view of the undersides where they would be proofed. I'd also make sure that this shotgun isn't loaded, since many of these muzzleloaders were loaded up once upon a time in preparation for immediate use, then put away and forgotten about; you can use a ramrod down the barrel to check, and if that rod doesn't go all the way to the nipples/vent holes, treat it as loaded.