A contemporary wrote recently on Facebook: "we had a physics master called Alec Dumbill (he was far from dumb). One of his favourite put-downs was: '[whoever], you're as dim as a Toc H lamp'. Never knew what it meant and still don't."
This struck me as disingenuous. John was not trying to conceal his age - he actually dated the exchanges (1956, if you must know) within his FB message - so I can only think he was pretending to appear less knowledgeable than he is, but I have no qualms about showing off. When we were growing up, there were still remnants of the Great War phonetic alphabet around. I can distinctly remember ack-emma and pip-emma, (a.m. and p.m.) being used in films and radio programmes. References to ack-ack (anti-aircraft fire) would have been understood almost into this century, even if the original derivation had been lost. A grammar school lad with any curiosity would have wanted to find out more, and I did.
The progress of phonetic alphabets is interesting. The clearest exposition I have seen is here. Being an army brat, I was fairly familiar with the Interservice set. The NATO alphabet, now in common use across public services as well as the military ("Juliet Bravo", anybody?), is clearly an advance in using names which are internationally recognisable, but some of the historical resonance has been lost in its adoption.
And Toc H? It stood for Talbot House, the brainchild of an Anglican clergymen, "Tubby" Clayton and named in memory of the nephew of a fellow-chaplain, Neville Talbot, during the 1914-18 war.
It was intended as a club house open to all ranks and conditions of men as an alternative to the numerous bars and brothels in the town of Poperinge. For many, Talbot House became a home from home. Its symbol - guiding light, perhaps? - was an Aladdin-style lamp to which the cross of Ypres was attached (the "Lamp of Maintenance"), seen dimly burning through the window of an upstairs chapel. The house's modern-day embodiment is described here.
Two years after the war, Clayton developed the concept of an international, interdenominational, association for Christian social service and, as we would say now, used an established brand name to give it instant recognition. Toc H continues to use the Lamp of Maintenance as a symbol. So the saying was probably originally "as dim as the Toc H lamp" and it was only later that the indefinite article came to be common.