The oldest phone boxes in Canterbury are of the style known as K6 (kiosk type 6). They have protection as listed 'buildings' in the city. Designed by Sir Gilbert Scott and introduced in 1936 as the 'Jubilee kiosk' to mark the jubilee of George V, they became the standard phone box for the next 30 years. The crown appearing above the door was the traditional Tudor crown. Early versions of K6 suffered from security problems - the cash box was easily ripped out and the small rectangular window panes could be prised out. There were reports of entire greenhouses made of small rectangular sheets of kiosk glass! The mark II version introduced in 1939 addressed these issues and incorporated rivets to secure the window panes. Examples of these can be seen at Westgate, Best Lane, St Peter's Place, and in The Borough.
On the accession of Elizabeth II in 1952, the crown was changed to the coronation crown. Examples can be found at Westgate (beside the Tudor crown version) and in Burgate. Further change came in 1955 when, in order to allow for a different crown for Scottish boxes, the crown was inserted into a slot. An example can be seen in Best Lane, standing beside an original Tudor version.
Sources: Johannessen (1999), Stamp (1989)