 When the conversion program was terminated, only 144 had been completed, leaving 20 gaps [5, 7, 22, 24, 27, 31, 33, 36, 38, 42, 45, 47, 52, 53, 55, 58, 59, 60, 61, 62] in the sequence.Each M car had 7 or 8 compartments, depending on the configuration of the original carriage; usually 3 compartments were allocated for smokers per car.When initially issued to service, most were designated ACM and BCM denoting either First or Second class accommodation respectively.