These coaches were built for the Southern Railway by the Birmingham Railway Carriage & Wagon Company and delivered to them between December 1947 and May 1949. They were made up into three car sets, the first ten with two semi-open brake thirds to D2124 (Comet Models S16K), which had one full compartment and a half compartment, or coupe. These sets were numbered 795 to 804 and referred to as ‘Type M’ or ‘Cross Country’ sets. A further 25 sets numbered 805 to 829 were built, but these consisted of the corridor composite and two of the D2125 semi-open brake thirds (Comet Models S15K) and were known as ‘Type L’ in line with other three car sets on the Southern. The formations were of this coach between the two brakes with the first class end adjacent to the even numbered brake coach.
Sets 795 to 804 first saw service on the South Western section but were all moved to the South Eastern section by September 1948. In 1954 the first eight sets went to the Oxted, East Grinstead, Tunbridge Wells lines, the other two sets being separated then combined with other coaches in 1957 to form sets 473 and 474. Sets 805 to 829 entered service between August 1948 and May 1949 on the South Western section. For summer traffic, some sets were strengthened with a pair of corridor thirds (Comet S2AK). Maunsell coaches may have been used but there is strong evidence that they were usually Bulleids.