BR Working Timetables Codes
The BR TT Codes are the code letters used for the different WTTs. Until the 1970s, there was duplication, even within the same Region, and codes changed frequently. From 1975 to at least 1995, a single set of 2-letter codes were used, the first letter referring to the office responsible for its production: Crewe, Glasgow, Liverpool St, Paddington, Waterloo or York.
A more detailed explanation is in course of preparation and will be published as soon as it is available.