What is the date on the cancel before the "X" for October ? seems to be a 18 too, if so it's probably a duplicate of the date, will make sence since the 18 is not centered with the Ontario
Side note :
In 1973 the Postal Code was probably implanted in Hamilton ( see the cancel )
Before the Postal Code was adopted, they was dividing the large city by Zone, at first 2 digits and in 1960 3 digits, but I don't see reference they was using it for Hamilton, only, Quebec, Montreal, Ottawa, Toronto Winnipeg, Vancouver and beside, they don't put the zone code on the cancel
your cancel with the roman number for the month was typical in 1973 , look at the pic.