inherited from the 'Big Four' companies were left unchanged, but with a prefix letter to show where they had originated from, as follows: A "D" prefix in front of the regional prefix indicated departmental (non-revenue earning) stock. Departmental stock (including locomotives) inherited from the Southern Railway carried numbers with an "S" suffix (indicating Service stock, an alternative term for departmental) which was deleted and replaced with the "DS" prefix. Former 'Private Owner' wagons, owned by industrial concerns rather than the railway companies, had a prefix letter "P" but were renumbered into a new series commencing at 3000. Some carriages and wagons built by British Railways to the designs of the 'Big Four' companies were numbered in their series and carried the appropriate letter prefix. BR then introduced its own number series, for wagons beginning with B. Initially most designs were based on, or similar to, those of the pre-Nationalisation companies. Arrangements from 1951 In 1951, the first production carriages built to British Railways' designs appeared, known as the Mark I. Numbers had a prefix letter (or letters) to indicate the region to which the carriage was allocated, and responsible for the maintenance. The inherited carriages then had a suffix letter indicating the company of origin. New carriages had no suffix: when Pullman Company carriages were added, their numbers carried both prefix and a suffix (indicating regional allocation), as did the 2 tier car carriers built for the East Coast services. The regional allocation letters used were: Wagons retained the existing prefixes indicating their origin, and new stock built to British Railways designs was given a "B" prefix. British Railways adopted the following numbering system for carriages and wagons built to its own designs (a small number of types built to pre-Nationalisation designs were later allocated numbers in this series, but only following rebuilding to new types). Note that in this table, reference is made to first, second and third class; from June 1956, second class was abolished and third class was renamed second class (and again to standard class in 1987). The few BR-design second class carriages (intended for use in Boat Trains since ships had three classes of accommodation) were designated as first class or unclassified. Numbers 100000 to 999999 were used for non-passenger rated stock (including wagons, vans and departmental (non-revenue earning) carriages), while internal user vehicles (stock used for internal purposes (e.g. stores) at specific locations and