Can any thing be done about that buttplate? Its so flat, I think I am looking at a trade rifle, instead of a DB shotgun. It needs a bit of curve to it, but not a crescent. It should be designed to fit in the pocket of your shoulder( when your arm is raised with the elbow out from your shoulder.) There should be some downpitch, since most men are full through the chest. No sense selling a gun that has a toe that digs into the shooter's pectoral muscles needlessly, so he has to spend money to have the stock changed.
I am also concerned that the double throated cock is not striking the frizzen at a proper angle to scrape steel from the face, rather than gouge it, and shatter flints in the process. The angle of the lower jaw of the cock to the face of the frizzen when the flint strikes the frizzen should be 60 degrees, where the bottom axis is the triangle lies along the bottom jaw, and the upper axis is a line from the top of the face of the frizzen to the point of impact with the edge of the flint. The POI is the center point for a protractor to measure the angle. The POI should be between 60 and 75 percent of the distance from the bottom of the frizzen's face, to the top of the frizzen. That permits the frizzen to be popped open as the cock scrapes steel from the face and throws it into the pan as the frizzen rises and backs out of the path of the sparks. If there is any evidence of skipping of the flint on the face of the frizzen, something is not fitted correctly. You should see a long scrape on the frizzen from the POI to just above the bottom of the frizzen where the frizzen pops back away from the flint to make room for the sparks to be thrown into the pan. With a new flint, the edge of the flint should be pointing into the center of the pan or just forward of it. If the edge is pointing beyond the front of the pan, when the cock is all the way forward and at rest, either a shorter flint has to be used, or an adjustment in the angle of the cock to turn the cock downward a bit is needed.
Adjusting a double throated military style cock is much more difficult to do than with a gooseneck shaped cock. You might want to keep that in mind before settling on a design. I would want a very good reason to be using the double throated cock these days. The steels used in casting lock parts is so much superior to the iron that was used to make parts for the original guns, that there is not reason not to use these steels and rely on their strength to produce a reliable cock that will not bend or break under normal use.