Marc,
That is a very American conversion of parts or possibly all of some sort of British ordnance gun. You said it is a rifle. Is the bore actually rifled? The fact that it has Ordnance proofs on it is intriguing. I don't know offhand of an English Ordnance small arm that had an octagon 43-1/2 inch barrel. The trigger guard looks like it is a modified English Ordnance type guard. Can you post a photos of the left and bottom views of the gun, and a top view of the butt, so that we can see the details of the trigger guard, the side plate, and the top of the butt plate?
Mike