Between 1923 and 1969, the BMMO built most of the buses it operated: up to 1940 these were called SOS (rumoured to stand for Superior Omnibus Specification), and some models were supplied to other bus companies associated within the British Electric Traction (BET) group, namely Trent, PMT and Northern General. After 1940, the vehicles were identified by the company's initials, BMMO, and supplied solely for the BMMO company's own use. Codes later used for buses were FEDD (Front Entrance Double Decker), REDD (Rear Entrance Double Decker), Coaches were initially classified "ONC", but later used a prefix of "C" – or "CM" for Motorway coaches.