I've got a few Alex Henrys, but only one that was a sealer, same caliber and the rowboat's letters carved in the stock and stamped under the forend. Have you checked yours in this location? I believe the stamping was done at the time of manufacture.
These sealers guns are rare as the salt water and conditions did a number on most of them, hence few survivors.
As shooters, they can be excellent, just need the right paper patch mould. I have another in 450/400 that Ralf Martini rebarreled, shoots 1 1/2" @ 100!
And classed as Antique!
~Arctic~