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.
