Wot a load of old bull - !
Please could someone explain what this square opening to the left of the entrance in this rectangular shell-proof pillbox is? It's clearly not a loophole as it is too high up and it has a dog-leg in it (i.e. there is no clear line of sight through the opening; see 2nd photo). The pillbox itself is on the GHQ line near Ford End in Essex. I think there are several with the same feature. Is it for ventilation?
You are correct, it is for ventilation
The dogleg vent was standard on the Vickers MG box
It's not clear from your photos exactly what type you have there, but based on the location, embrasure in the blast wall and the fact that it is rectangular, it could be an "Eastern Command Type", which is a variation on the VMG Box.
The diagram there doesn't show the vent, but from experience there are significant local variations in implementation.
It is an Eastern Command Type.
These dog leg ventilation ducts can also be seen on some of the T.28s on the GHQ Line in Cambridgeshire.
Thanks all for the information.
As has been stated, they were common to Medium Machine Gun post typically the Vickers gun fired from a mounting. Considering that these guns (water cooled) were capable of sustained firing, the amount of gas blow-back from the muzzle was a liability in enclosed spaces especially if there was a wind blowing against the embrasure.
The ventilation duct would assist the escape of warm air at ceiling level and create some ventilation. The dog-leg was intended to defeat furtive grenade or small arms fire against the occupants.
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