Designated Class 20 the D8000 series of locomotives were built and delivered in several batches between the late 1950s and the late 1960s with the first 200 of the build being numbered in the rage D8000 to D8199. The final build were numbered from D8300 onwards due to the D8200 series already being taken.
On renumbering to the TOPS series the Class 20s in the number ranges D80XX and D81XX were renumbered in sequence except for D8000 which became 20050 at the end of the second of the initial batches ordered displacing D8050 to become 20128 at the end of the disc headcode variant. D8128, the first built of the headcode box variant in turn was displaced to become 20228 at the end of the numbering series.. Those numbered in the D83XX number range became 202XX on renumbering in sequence.
After renumbering to the TOPS series the Class 20s were numbered in the range 20001 to 20228.

Two Sub Classes have since been created, one of which, the 20/3 series, has been used twice. The first 20/3 Sub Class was for several Class 20s with modified brake equipment and these were renumbered into the range 20301 to 20308. All eight were subsequently returned to their former identities.

20023 became 20301 before reverting to 20023
20059 became 20302 before reverting to 20059
20134 became 20303 before reverting to 20134
20168 became 20304 before reverting to 20168
20172 became 20305 before reverting to 20172
20173 became 20306 before reverting to 20173
20194 became 20307 before reverting to 20194
20196 became 20308 before reverting to 20196