I see the D mark ..... It's almost certainly .577 calibre.
Am I reading the 'D' stamp as 'Dominion' correctly?
What would the value be on something like this?
Actually, it
can't be .577 caliber - or in any event is extremely unlikely to have been bored out that much - since the outside diameter of a Martini-Henry barrel at the muzzle is only about .700" reaming it out to that extent would leave barrel walls only about .015" thick at the muzzle end!
The "D" mark is not a Canadian military mark, and does not denote "Dominion" that I am aware of. If you are referring to the "D" marked on the right side of the "nocksform" (the swelled, flat-topped area of the barrel immediately in front of the receiver) that is almost certainly an Armourer's mark of some sort. My references indicate that a D stamped on a barrel denoted "new standard of finish for bore" - i.e. it could signify that, prior to being converted to a smoothbore, this rifle may have had its bore and rifling "refreshed" because of rusting or pitting in the bore (with the mark remaining from that process), or it could refer to the ultimate conversion to a smoothbore.
The early Canadian property mark you may be thinking of was in fact the letters "DC" (denoting "Dominion of Canada") which normally looked like this, and was stamped on the butt of the wooden stock on military rifles -
Occasionally one sees a "DC" mark without the diamond-shaped border around it ....
To be honest, I have no idea of value of such a smoothbored Martini-Henry in the condition evident from the photos - there isn't much demand for them, since collectors and shooters generally want something with good, intact rifling, in original military configuration. If it is fully functional, I'd guess maybe $300 or so ....