When is a postbox not a postbox?

Wandering down a track near Logierait, taking pix of the mist and a frozen field, I came across this anachronistic Georgian cast-iron post box, set into an old stone gatepost. Post boxes should be red, and they don't generally exist on farm tracks. It might have remained a mystery had not a Land-Rover stopped and the driver explained that the box had been located on a bad bend in the village until being hit by a wayward vehicle, and replaced with a modern one in a more favourable location. The driver had offered to purchase the old one, but Royal Mail has no procedures for such, so he persuaded them to let him set it at his road end, to save the postie from having to drive all the way down to the farm with his mail. Painting it green was supposed to prevent accidental posting, but he does occasionally get misplaced mail from the Irish and Germans.... Not the best photo (from my phone) but I liked the back story.