While the ebay link is now dead, and we have no picture, I do recall the fitting discussed. The Australian versions were indeed made by Howard's, but they weren't designed in Australia for the Inter Mk1 apparently.
A UK Ebay seller is selling a copy of a British manual from 1953 (
https://www.ebay.co.uk/itm/50-cal-L1...gAAOSwo6lWQ1yc) that shows that these roof mounts were a British design, for a range of vehicles and received the nomenclature of Mounting, MG .50 AA, L1A1 and Vehicle Stand, L1A1.
Seems we adopted the mounting from the Brits and included it in the design of the Truck, 2 1/2 Ton, No1 Mk1. In this pic of a No1 Mk1 we can see the ball fittings on the roof