The red pillar box as we know it today first appeared in the UK in 1857. Early boxes varied in design, and weren't standardised into the cylindrical boxes we know today until 1879.
In 1866 a hexagonal shaped Post Box designed by an architect & surveyor by the name of John Penfold was the very first standardised public post box to be erected in London. This became commonly known as the ‘Penfold Box’. It also displayed the Royal cipher of Queen Victoria 'VR' which stands for Victoria Regina.
This particular box is a rare example of that very earliest design.
To find the cache you must count the 'balls' around the cap of the pillar box. This will be x.
The cache can be found at N51.29.(11*x)+34 W000 11.(4*x)+6
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