total capacity of each unit was accordingly 386 seats, except units 5005 and 5220 which seated 374. Third class was renamed second class in 1956, and the car designations became driving motor brake second open (DMBSO), trailer second (TS) and trailer second open (TSO). In 1960, the first British Railways design units appeared. Intended to replace the 1925 design Southern Railway suburban electric stock, these units were based on British Railways Mark 1 coaching stock with a different body profile and underframe length from the earlier Class 415 units. The first two units (5301/02), however, were composed of Mark One profile Driving Motor Brake Seconds and Southern Railway profile intermediate vehicles. Unit 5303 was the first to sport intermediate trailers of Mark 1 profile. Two units differed from the rest of the batch in featuring B5 (S) bogies to enable use on peak hour commuter trains to Eastbourne. Reformations over the years, following accidents and incidents, led to a few units becoming composed of a mixture of original design stock of Southern Railway outline and the later, Mark 1 based, British Railways stock. In the earlier, Southern-style 5001-5053, 5101-5260 series, most units (all of which were one class only) comprised a driving motor open saloon including brake at each end of the set, sandwiching a trailer open and a high-density (6 per side) trailer ten-compartment vehicle with access from the passenger doors only; there was no gangway down the coach. A very small number of these 4-EPBs comprised either two open-trailers or two compartment-trailers. A small number of the trailers had been built as 'composites' – a mixture of First and Third Class – and were later fitted out as 9-compartment one-class vehicles but with the former 1st accommodation still identifiable with extra-wide compartments. In the mid-1960s, a number of compartments were marked as women-only, because of the density of cigarette smoke at rush-hour in the general compartments, but as these were not well-regarded they were withdrawn in the late 1960s in favour of non-smoking cars, marked by red triangles on the windows. At first one car per set was allocated to non-smoking, but soon one driving car and one trailer in each set were the norm. The production vehicles in the BR series 5301-5370 had slightly higher capacity motor coaches, identical vehicles at each end of the set, with an internal partition splitting the saloon into two smaller ones,