WM Holden , based on Cooper T20 ( WM stood for Waggott Myers )
You've got it, and the point, which brings you ever nearer to Pro level. The car had a modified Cooper T20 chassis fitted with a Holden engine with Waggott ohv conversion.
Nobody seems to know what Moss was doing there. The track is known as the Parramatta Speedway, a quarter-mile dirt-track in Cumberland Park. There are images on the web of Moss sitting in the car, and driving it round the oval, but the car was built for road racing - it must be around the time of the Australian GP and other races in the winter series. How and why Myers persuaded him to drive the car is presumably lost in the mists of time.
Well solved. I've never seen these photos before, and know nothing about the event.