SE27 West Norwood: why this London postcode has the number 27

Last in the alphabet, after Sydenham

West Norwood is SE27 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: West Norwood comes after Sydenham (SE26), and is the last of them.

See SE27 on the map