OK, found it. (Thought I'd better check...!)

Henry swapped his hammer to the left-hand side after criticism from the Military Select Committee, a body formed to adopt the new British service rifle, that the hammer could get in the way of loading the cartridge for a right-handed man. Henry then continued manufacturing left-hand hammer rifles for some time after that, even for commercial sporting purposes, however he switched back sometime later as the right-hand hammer was deemed to be in 'a more natural position'. See Winfer, Vol.1, pp 76-77.