It looks like one of the many variations of a 1953 Mac Co (McGrath Trailers) Australian medium 2wh trailer. Different versions have the chassis channel facing outwards or inwards, but all are rated at 1 ton.
See bottom portion of
THIS THREAD
Although the hitch has been changed for something more convenient to tow around the farm (Hey, pintle hooks are the most convenient things going!), this still looks to be a sound original base. Hang on to it! Nice to see another around.
The tyre size (as with many McGrath trailers) was 7.50-18, an unusual size today for road tread, but still available for tractors and implements.