SW20 West Wimbledon: why this London postcode has the number 20

SW20, West Wimbledon, is not in the Post Office's list of March 1917, which ends the South West at S.W. 19, Wimbledon. Added later, it took the next free number; it is in the Post Office's district list of 1980. A 2017 study calls NW11 and SW20 “added later out of sequence”. The year it was created was not found.