|K6 Phone box|
|K2 Phone box|
K1 was the first design; Historic England lists six (Grade II listed) none of which are on the streets of London. The K1 dates back to 1920.
The K2 followed in 1924 as the result of a competition won by Sir Giles Gilbert Scott. The K2 is easily recognisable by its equal sized panes of glass.
The K3, K4 and K5 were not widely distributed (if at all in the case of the K5) - and the next significant development is the K6. The K6 has one wide of pane with two narrower panes either side and is somewhat smaller than the K2.
All phone boxes had a crown above the 'telephone' representing the British Government; at the time phone boxes were run by the post office who were, at the time, part of the Government.
K2 phone boxes have an embossed crown which doubles as a ventilation hole. On K6 phone boxes the crown is represented in bas-relief and a ventilation hole is provided elsewhere.