The Toc H symbol was an old fashioned oil lamp, not known for it's strength of illumination, hence the phase "dim as a Toc H lamp" to describe someone who's not the sharpest knife in the box.
Toc H is a charity originally for soldiers that was started by Chaplain Philip Clayton on 11 Dec 1915 as a 'soldier's house' in Poperinge near Wipers (Ypres.) It was proposed as a place where the lads could get a brew and a wad when away from the front.
Originally to be called 'Church House,' it was realised that the Toms would have avoided it like the Badge Man on a Friday morning.
It was named Talbot House in memory of Lieutenant Gilbert W L Talbot, aged 23, who was the brother of Padre Neville Talbot.
The name Talbot House soon became known to the troops of the Ypres Salient as 'Toc H'; ('Toc toc' being the phonetic for 'T' (now Tango) at the time.