The first post-boxes in the UK were introduced in 1854 in the Channel Islands thanks to the then Postmaster General, author Anthony Trollope. They were sage green but changed in 1874 to the now famous pillar box red.
With each change of monarch the royal cipher on the boxes changed throughout the country until the death of George VI when a problem arose.
Queen Elizabeth is Queen Elizabeth II of England and Wales but not of Scotland. At the time of the first Queen Elizabeth, James VI was the Scottish monarch.
As a result the Scottish pillar boxes installed since the early 1950s have simply borne a crown and no royal cipher.
One of the lamp post-boxes on the Isle of Lewis has had its Royal Mail sign painted over and a new red crown on a silver background plate. This is the first one of these I have seen.