From the Wikipedia article :
Walthamstow was a local government district in south west Essex, from 1894 to 1965, around the town of Walthamstow. It was within the London suburbs, forming part of the London postal district and Metropolitan Police District. Its former area now corresponds to the central part of the London Borough of Waltham Forest in Greater London.
It became a Municipal Borough in 1926.
Some municipal history can be found here .