postalmuseum.org

oRJME TIL in Britain, the Victorian's Added Secret Messages to Their Postcards and Letters.
1aXx7 TIL postboxes in the UK bear the 'royal cypher' (initials, basically) of the monarch on the throne at the time they are installed, and have done since Queen Victoria's reign - meaning you can very roughly ascertain how old a postbox is just by looking at it
ADwa7 TIL that there are ciphers on UK postal boxes that show the reign of a Monarch of when the pillar box was established in the area.