These ads give a nice indication of production rates over 1958 and 1959. 150,000 mowers between February '58 and March '59!:

February 21, 1958 - Daily Mirror
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February 9, 1958 - Sydney Morning Herald
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November 23, 1958 - Sydney Morning Herald
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March 22, 1959 - Sydney Morning Herald
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These figures could prove quite useful in working out a date/serial number correlation also.