Hi Chris,
The referenced “A&T” likely means Adams & Tate the barrel makers. You may find a very small 'A&T" stamp on the barrels ahead of the flats and behind the forend hanger.

I do hope you manage to get a bit more expanded information from Watson Brothers. As you mention, some of the other gunmakers (Rigby, Hollands, Dickson, Westley Richards, William Evans) are very good at providing extensive information. A couple others are not quite so forthcoming.

- Mike