My work colleague has found this object in a field adjacent to an old RAF base on the Lincolnshire coast. It appears to be made of iron and has a silvered finish. The diameter is 11.5 inches and it's extremely heavy at 9.8kg. There is a lip around the edge which suggests that it once fitted inside a tube. We wonder it's a relic of some form of ordnance and would be very grateful for any suggestions as to what it might be. Any clues?