It looks just like the Shadow in Frakor's picture on post 7.
The cylinder has the locating peg hole in the place for the later sight and not in the place of the earlier "bridge" type sight, so I assume that the cylinder and sight have always been together. The lack of any Bsa name stamping or model stamping, makes me think it has come out of the factory like that probably using up bits that were lay around.
I don't like the thing anyhow and am giving it to someone.