FW3/23.

As I am reliably informed. 

This pill box type is on one of our old railway lines and today I've had a meeting with the Wildlife Trust to see how we can work together to restore it and also use it as a bat roost. It is fairly remote and it would be really nice to recognise its heritage along with giving it a new lease of life.

Apparently this one departs from FW3/23 specification but most strongly resembles that design. Fascinating to hear how it was expected to be used and chilling to hear the outcome of those who might have had to use it if the worse happened.