Wall boxes are a type of post box that is set into a wall whereas our pillar boxes are free standing. They first began to appear in more rural areas in 1857. The boxes bear the initials of the monarch reigning at the time they were made.
I came across this Victorian one in the Cotswolds village of Lechlade and it is still in use today
This Georgian wall box does not have the roman numerals depicting which monarch so I am assuming it dates from George V as the regnal numbers were only added from George VI reign onwards. (George V reigned from 1910-1936). I came across this box in the village of Penshurst, Kent.