SE20 Anerley: why this London postcode has the number 20

First in the alphabet, before Dulwich

Anerley is SE20 because of the alphabet. SE19 is Norwood, a district office since 1880, and in the Post Office's list of March 1917 the offices after it follow by name: Anerley comes first, before Dulwich (SE21).

See SE20 on the map