E17 Walthamstow: why this London postcode has the number 17

Walthamstow is E17 because of the alphabet. E1 is the Eastern district office, and in the Post Office's list of March 1917 the offices after it follow by name: Walthamstow comes after Victoria Docks and North Woolwich (E16) and before Woodford and South Woodford (E18).