Nothing to do with MDN and his weekend activities, but was instead an artillery OP training aid from the days before Invertron and whatever that mortar version was called that had you attacking the motorway past Stirling Castle.
It consisted of something that looked a bit like a model railway layout made of Hessian, that simulated a bit of terrain from a map. It was laid over a chickenwire frame about 6' off the ground, with grid squares marked on the floor underneath.
The effect of a shell landing was created by a chap who pushed a trolley with a "smoke" generator under the model, lining it up with the grids painted on the floor.
The "smoke" was made by passing ammonia vapour through conc sulphuric acid or somesuch. Quite why Health and Safety stopped us using it is beyond me.
A similar effect can be achieved with bits of cotton wool, but that gives no excuse for getting nigs to inhale deeply of ammonia fumes.
These days, it's all computer generated in the Invertron AFCT to suit the Playstation generation.