This was given to me by a friend who found a box full of them while filming in a disused MOD base on Portsdown hill overlooking Portsmouth. Its made from two white cardboard discs with a clear plastic indicator and held together with a brass hinge.
So dialling the arrow at the bottom to a modest 5 Megaton bomb, and moving my indicator to my bunker's location at 4 miles from the centre of the blast, I see there is heavy to light damage to houses. Surprisingly, no one is dead but 10% are trapped in rubble. Of those not trapped 6% are seriously injured. I am further inconvenienced by "severe vehicle displacement".
Official advice therefore, as it tells us on teatowels and mugs up and down the country is to "Keep Calm and Carry On".
Severe Vehicle Displacement
You can find this and many other interesting objects donated by the curators, at Mark Fisher's Found Objects blog.