The red pillar box as we know it today first appeared in the UK in 1857. Prior to this, designs varied from area to area. To begin with these were hexagonal in shape, but were standardised into the cylindrical boxes we know today in 1879.
You can tell in which era a box was constructed as it bears the initials, or 'cypher', of the reigning monarch at the time - eg VR (Queen Victoria 1837-1901), GR (George V 1910-1936), EIIR (Queen Elizabeth II 1952-2022).
In each case the 'R' is Latin for either 'Rex' for a King, or 'Regina' for a Queen.
King Edward VIII came to the throne in January 1936, but had abdicated by December that same year. As such, very few pillar boxes containing his cypher were made, and it is thought only about 150 still remain nationwide. This is one of them.
At the pillar box you will be able to see how much a book of stamps cost back in the day. Call these two numbers AB.
The cache can be found at N51 30.A(B+6)(A-1) W000 03.(A+1)(B+2)(A+2)
If you are interested in unusual pillar boxes, then check out my other caches in this series:
Special Delivery #1 - GCACP55
Special Delivery #2 - GCAR9WC
Special Delivery #3 - GCARPBA
Special Delivery #4 - GCAVWE1
Special Delivery #5 - GCAXDX6
Special Delivery #6 - GCB4GFY
Special Delivery #7 - GCB118Y
Special Delivery #8 - GCAXE0N
Special Delivery #9 - GCB2ZV0
Special Delivery #10 - GCB359X