Manfred,
WG/O, WG/R, WG/T, and WG/T/S have all been reported fitted to S5/S6/T6 models, but so far it has not been determined what was different about the WG/O gearbox. The WG/ gearbox was introduced on the 600 EW of 1927, internal to the factory seemingly designated as the SB27. However the later designation SB I agree with Eddie as certainly a mistake for S6, as these gearboxes were subsequently fitted to that model.
Gearbox prefixes are also listed in the tables for identifying Douglas models, which can be found on this Forum.
-Doug