These metal plates stand in Palace Street (Cathedral side), Watling Street (at the corner of Rose Lane), The Borough (at the entrance to King's School), and in Abbot's Barton Walk. Each is headed with the Royal crown and initials GR, and, by means of an inserted metal strip (all five are lost), gave the distance in feet and inches to a main junction point of GPO phone cables. The GR dates them to the reign of George V ie 1910 to 1936. The arrow below denotes crown property as GPO were then in effect a government department - not as we at first guessed an Ordnance Survey bench mark which is a much broader arrow.
When telephones came to Canterbury in 1892, the City Corporation wanted to avoid disfiguration of city streets by large numbers of telegraph posts. The Post Office resisted, and the case went to court. The Corporation won, and the Post Office had to site its cables underground.
If you know of other similar plates do please let us know. We hope to convince English Heritage that those we have spotted should be listed for their protection.
Sources: with thanks to Paul Crampton and Malcolm Mercer; see Canterbury Cathedral Archives CCA-CC-BB/183 for the court case against the Post Office