We had a family cottage in McKellar ON near parry Sound and Ardbeg and Whitestone was just up HWY #124, They had the last manned Fire Tower in use in Onatrio, retired in the 70's in favour of aerial spotters. I think the population was something like 800 when I was a kid.
International had the R-series (1950-67) with great big inline six gas engines
and the V-series (l955-68) with great big V8 gas engines. The fire truck in the ad is a V-series and could have come with a 401, 461 or 549 c.i.d. gas V8. The cab dated all the way back to 1950.
I forgot, they also had the heavy duty M-series with the 1950 cab all the way to 1971.
The truck series that replaced it in 1969 was the Fleetstar A, using the new 1969 cab shared with the pickups. Power came from gas big sixes (406, 450, 501), gas big V8s ( to 549) and various diesel engines.
They were replaced in 1978 with various S-series trucks.