One of the best locks for doubles as well as one of the best actions have been designed by Thieme & Schlegelmilch, the Nimrod sidelock, the Nimrod boxlock and the Nimrod action. These great developments are now made again by Ziegenhahn & Sohn from Suhl. A short description can be read on their website :

http://www.ziegenhahn.de/en/produktion/flinten/boxlock-anson/

The Nimrod sidelocks are a construction most probably superior to the H&H sidelocks.
Maybe even better were the Sauer sidelocks, but unfortunately they are not produced no more and I am sure Sauer will never make such a quality again.

Kalunga