I would think about August 1955 the model 2 crank cases would be out.
7,864 has a model 2 crank case and 6,883 has a model 1 crank case , so somewhere in between would be the start of the model 2 crank cases ,I'm thinking in the 7,000 range and not the 6,000 range.
The production numbers obviously increase in later years, example 2,500 mowers a week in March 1957.
A few lowish numbered model 2 Rotomo mowers I've seen are 17,420 QLD / 10,219 NSW / 23,899 NSW
In Victoria a Rotomo that was found 25 min or 21 km from 440 Burwood Road, Hawthorn Melbourne (Ray Tijou’s mower shop) was number 18.056.(see pics below)
