So we know that the gun had to have been made some time between 1921 and 1933. It was most likely to have been produced in London, though Birmingham should not be ruled out.
Because they were selling their own brand of cartridges, the gun is likely chambered for them. Do a chamber cast to be 100% sure of the chambering. You can use cerrosafe or bee's wax for the cast.
Post pictures of the proof marks.
Have a good gunsmith who knows these old double rifles take a good look at it. They will be able to properly clean the action and should have a bore scope to inspect the barrels.
Pete
