There was never a diesel prototype built of the light tractor Greg, the one you've pictured, complete with the original grey motor, is the only one ever assembled by Rolloy. Not long after the prototype had been fully tested at the government farm at Werribee, Holden had the red motor in production but there was no point in updating the prototype's engine at that point since the testing was completed. The diesel version was planned, using a V4 air cooled design - it was similar to the small Deutz engine of the time if I recall correctly - but it never got beyond one cylinder block on a stand. The prototype you've photographed was donated to the Science Museum before its designer's death.
