 The four exceptions were carriages 49B, 51B, 20AB, and 79AB (108B from 1911).These cars had the same external dimensions, except that 49B and 51B were originally built with eight compartments of 6'1 5/16" and a ninth added at 6'11½"; 20AB and 79AB (108B) originally had seven compartments - three First class in the centre at 6'11" each, flanked by two Second class compartments of 5'9¼" each; later two standard 6'0 11/16" compartments were added at one end.By the time carriages were being withdrawn from steam-hauled service for conversion to electric train trailers, the eventually-converted fleet comprised 25 x 1st-class cars in the range 1A to 149A (very few consecutive, but the majority numbered 100 or higher), and a further 89 2nd class carriages, most in the number range 1-222 plus 129B and 134B.