LMG mount in use
 

Often mistaken for a lifting device for spare tyres, the AALMG (anti-aircraft light machine gun mount) on CMPs and other Australian vehicles was a counterbalanced device, usually located over a hatch on a CMP and was suitable for guns such as bren.

Here's a shot from the information board at the Army Tank Museum at Puckapunyal of a White Scout Car modified as an armoured observation post.
The interesting detail of course is the AALMG.

http://oldcmp.net/images/us/white/135013.jpg

LMG
 

Hey Keith,
Does the hook on the counter balance go under the top handle or under the barrel?

Hook
 

My eyes aren't that good!

But it looks like it's under the barrel with part of the hook around the handle.