This confirms Your Ad 12 August for the Rotomo 1 with the new 125 cc engine to be spot on the money Unkie!
Quote from the Victa Story
"By April, 1955 it was ready to offer its new engine to the public and by June, eight weeks later, it was producing sufficient to swing its whole mower production completely away from the old engine"
So production was in full swing in June. So allowing some time for transport and dealer Set up,this Ad would almost certainly be the First!
Here is the Sales for 1956/7...
"Mower sales during the second stage of Victa's marketing plan showed a substantial increase over sales made in the previous two-year period. Sales rose from 35,577 to 59,974 (in 1956-57), and then to an all-time high of 66,742 (in 1957-58)" these are
So approximately 60,000 mowers for 56-57....The next sales Detail shows the 148,000 sales total including the exports..