London Passenger Transport Board

The London Passenger Transport Board (LPTB) was the organisation responsible for public transport in London, England, United Kingdom, and its environs from 1933 to 1948. In common with all London transport authorities from 1933 to 2000, the public name and operational brand of the organisation was London Transport.
London bus route historic numbering.
Historically, bus routes run by London Transport were grouped by the type of service that they provided.
The 1924 London Traffic Act imposed a numbering scheme known as the Bassom Scheme, named after Chief Constable A.E. Bassom of the Metropolitan Police who made it. Changing and short lines used letter suffixes. The numbers were grouped also by the company that operated the route.
The numbering was revised in 1934 after London Passenger Transport Board was formed: