Anti aircraft defence warning sign WW2 River Severn estuary UK

I found this wooden sign very many years ago deep in a high dry old sandbank on the edge of the river Severn UK near where the anti aircraft guns ...maybe also the z battery rockets.?.....were in the 2nd World war.It is 45.7 cms x 38 cms and about 1 cm deep red painted on plywood.I have researched/asked media/historical societies etc but cant find anyone who knows about this.I consider it obscurely beautiful .Its been through many house moves ,kept aired and dry ,not pristine when I found it though!I want it now to go to someone who will treasure it too.