[Humber-One-Ton Tom]

Hi Richard-
further to Clive's comments, I have seen one of these gun mounts, but sorry no photos.

Basically it is a metal ring about 4" deep and the same size as hatch opening, suspended almost 2 foot above the roof hatch - this has four metal 'legs', each one is like a 'V' shape made of metal rod - a cup at the foot of each of the legs fits on the balls and the top ends attach to the ring.

I think that a bren gun or similar can be attached to the ring. As far as I could tell it does not swing on a pivot, rather it is on a fixed mount facing out from the ring, and this mount runs around the ring like a curtain track.

And of course the seat on that side can be raised so you sit with your head out of the hatch - but I guess you would stand on the seat to shoot.
Does this help you picture it?
Tom
Portsmouth, UK