What's the King on Your Postbox? Royal Cyphers on the Post Boxes of the United Kingdom in OpenStreetMap

British post boxes carry the royal cypher of the monarch who reigned when they were made, so the boxes on a street are a dated, if unplanned, record of when it was furnished. I use OpenStreetMap, whose volunteers record a royal cypher tag on many boxes, to count the surviving boxes of each reign across the United Kingdom. On 28 October 2026 OpenStreetMap held 90,190 UK post boxes, about 78% of the 115,000 that Royal Mail reported in 2002, and 61,435 had a cypher of a known reign. Elizabeth II accounts for 59% of these; Victoria for 3,614 boxes (5.9%), most of them wall boxes; and Edward VIII, who reigned for less than a year, for 143, which is 84% of the 171 survivors identified by the Letter Box Study Group. In Scotland 99% of post-1952 boxes with a recorded cypher carry the Crown of Scotland, the outcome of the Pillar Box War of 1952 and 1953. Among 131 well-mapped towns, the share of boxes from before 1910 ranges from 48% in the City of London to almost none in post-war new towns, and in a small check of 16 towns it rises with the share of today's population already present in 1911 (Spearman's rho = 0.63). Cypher tagging is uneven, from 30% to 100% of boxes by area, but I find little sign that it favours old boxes. The results describe mapped boxes, not every box, and a cypher dates a casting rather than an installation. Interactive version: https://tristan-hotham.github.io/postbox-kings/. Data and code: https://github.com/Tristan-Hotham/postbox-kings. Blog post: https://blog.tristanhotham.com/2026/10/28/postbox-kings/. The data files derived from OpenStreetMap are released under the Open Database Licence (ODbL 1.0); the paper and other material under CC BY 4.0. Contains OpenStreetMap data © OpenStreetMap contributors; contains OS data © Crown copyright and database right 2026.