This is the short underframe version of the BR Mk1 non-gangwayed third, having nine compartments with six-a-side seating. They were reclassified as seconds from 1956. All were fitted with B1 bogies. The last batch was built to ‘Metrogauge’ for use on the Widened Lines to Moorgate. This involved the roof vents being placed 1’8″ from the centre line rather than the standard 1’0″.