postalmuseum.org

Y7vev TIL that the acronym RMS stands for Royal Mail Ship. Originally, it was "Royal Mail Steamer," signifying that the Titanic was hired to transport mail. The Postal Museum's collection of plans below illustrates the location of the Post Office and Mail Room, which were located on decks.
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