The top Right hand hole which is the larger of them all, if I remember correctly is used in a GS (and civvy 2 and 2a) vehicles to locate the panel that houses the vent controls. In a gunbuggy of course the vents are sealed in the closed position and the wipers relocated, therefore this hole wouldn't have been used in a S2 buggy.
Onslow only has the one screw in his map light panel on the top and it utilises this hole. There are two on the bottom. Daisy the 88"GS uses two holes, one sharing the vent control panel, and the other to the left hand side of the panel.
Onslow my gunbuggy has a pipe (swivel) mounted on the door hinge nuts (captive in the firewall) and on this was mounted an arm that came across the vehicle to a position in front of the crewie in the left seat, where there was another swivel upon which the GPMG mount was fitted. This is the best shot I have of what remains of it today and its raining at the moment, and I am not going to get soaked just for you Stuart, you will have to make do with this one.

My firewall, when I got the vehicle (first rego out of service) had one hell of a depression in the centre where the constant pounding of the 106 barrel over the most of its life had bent it into a banana shape. Not only was the top rail bent but the support under the barrel lock had transferred the impact to the lower level. Possibly one of the reasons that it was converted to a MG vehicle in its later portion of its 32 year service life. A bit of a job to straighten it all up.
Regards
Glen