The base was used as mount for a 29mm Spigot Mortar or "Blacker Bombard". The Bombard was devised during the start of World War 2 by Lieutenant-Colonel Blacker as a cheap, easily produced weapon to replace ordnance that had been lost during the British evacuation from Dunkirk. It was used by the Home Guard and airfield defense units between 1941-1944.
The Mortar sat upon the steel pin at the top of the concrete base and would have been surrounded by a concrete weapons pit which would have provided protection to the three man gun crew. It would fire a black powder propelled, fin-stabilized, 20 lb anti-tank mortar bomb which contained a high explosive charge.
One nasty characteristic of the mortar was upon impact with its intended target, the fins would often be projected back down the original trajectory towards the firing crew!
The Bombards' effective range using the anti-tank bomb was only 100-150 yards, which meant it had to be used at very close range. It could also fire a 14 lb anti-personnel bomb with a safer 500 yard range.