Wall box

Wall boxes are a type of post box or letter box found in many countries including France, the United Kingdom, the Commonwealth of Nations, Crown dependencies and Ireland. They differ from pillar boxes in that, instead of being a free-standing structure, they are generally set into a wall (hence the name) or supported on a free-standing pole, girder or other stable structure. In the UK, UK Dependent Territories, Colonies and former Colonies and in many former British Empire countries, wall boxes usually bear the initials of the reigning monarch at the time the box was made. The first UK wall boxes were erected in 1857 in Shrewsbury and Market Drayton.

Wall boxes are normally made of cast iron and are fabricated in two large castings with a third casting for the door. The rear part of the box which is set into the wall is cast as one piece, rather like an upright coffin. The whole front of the box including the aperture, royal cipher and the collection plate holder, are cast as one. There is a large opening in the front casting into which a hinged door is inserted. It is held in place by steel pins at top and bottom which locate into machined holes in the front casting. A secure 5-lever Chubb lock is fitted to all wall boxes made after 1870. Previously, narrower locks from the firm of Nettlefields were used. They are not interchangeable and this causes problems when keys are lost and locks have to be changed.

Initial examples were fitted with a simple wire tray internally over the base to raise any post off the lower surface. This was necessary because water seeped into the box through the side seams and condensation could also form on the inside, making the base rather wet. Later modifications saw this wirework extended to form a vertical barrier across the whole inside front of the box. This prevented mail from cascading out onto the ground when the door was opened for clearing the box. By 1879, UK wall boxes were also fitted with a drop-down hinged metal letter chute to help guide the stored mail into the waiting sack or basket during clearance.

...
Wikipedia