Had a good long think about the whole saga of the single versus twin
gun turrets today and as usual the light bulb moment (aka penny drops)

happened. I have a feeling that Mac's comments were more referring to
the fact that the Mk IIIa/Mk V (twin Brownings)were the standard models circa 1944,
in the Upper Atlantic.
The West African units were saddled with the Mk III with the single
VGO.303 MG obviously sans the midupper turret. The Luftwaffe were not a threat
in West Africa, as perhaps the area nearer the UK/Bay of Biscay etc.
This then fits in with your coments and photos of a single gun turret.
All is good, I know what I need to do now
Thanks for your help
Regards
Alan