When built the Birmingham RC&W type 3, as they were then known, were allocated the numbers D6500 to D6597 in the 1957 number series. Under the Eastern Region classification system they were classified D15/1 (latter15/6) for the standard locos (D6500-85) and D15/2 (later 15/6) for the narrow bodied examples (D6586-97).

When the first list was drawn up for the TOPS numbering system in 1967 it was proposed the class would be split into two, class 33 for unmodified locos and class 34 for the 19 locos then undergoing conversion to push-pull working. These 19 locos were picked randomly from the standard gauge series. Classification was revised in 1969, all locos would be class 33 and subdivisions were proposed 33/1 for standard locos, 33/2 for push pull and 33/3 for narrow body. This was further amended in 1972 with standard locos being 33/0, push-pull as 33/1 and narrow body 33/2. TOPS renumbering took place between 1973 and 1975, standard locos being numbered in order as far as possible ignoring the two withdrawn locos and 33/1 converts. Push pull locos also followed their numerical sequence, the narrow bodied locos were always a discreet batch.