"The trailer is a Brockhurst 1 ton trailer usually towed behind a 2 1/2 tonner. They came in to the Army back when we had the Humber 1 tonners. They remained after the Humber was long gone and stayed as is with the tightly curved mudguards until late 60s/70s when the guards were replaced by a squared off type to facilitate use in muddy terrain where the mud stuck to the tyres and jammed the wheels."
I think you'll find the Australian Army's trailer was built by Corio Air in Geelong, Victoria, and is an Australian copy of the English trailer, but with local manufacturing techniques employed. There was an interesting thread on another forum about the shape of the guards, which concluded that the shape was changed due to the round guards jamming with mud and the restricted access for cleaning - very much the reasoning behind the Land Rover 'squared' guards. That Corio Air 1 ton trailer thread also had some images of the holes used for the round guards, filled with bolts, and the square guards fitted.
I've not come across anything official with regard to the replacement of the guards with the squared ones, so I'd be interested to hear if anyone else has. The square guards were professionally manufactured, so appear to be an official mod: hence, somewhere, the data supporting the change exists!
Mike C