This post box catches my attention when I walk into town. It's out of service and it's painted green, although you can see flecks of red paint underneath.
Green was adopted as the standard colour for the early Victorian post boxes.
However GR is for the Reign of George V 1910-1936.
Basically I can't answer this question and I've googled extensively.
The only possibility is that green boxes, usually without a slot are used for the local postman to store letters in on his round to save carrying them all together.
All I know for sure is that this post box isn't used anymore.