According to those who know more than I do about tractors, the UFO (Unidentified Farming Object) seen at the Vintage Harvest event in Carterton, is a 4WD Massey Harris built around the 1940s. Apparently they were originally manufactured with steel wheels, so this one has been converted. They were not very successful as the axle housings were prone to failure, due to not being built heavily enough.
Here is a pristine example I found elsewhere on the net.
They were known as the Massey Harris GP and were introduced in 1930.