HS im pretty sure youd be correct. I think the 1600 even older than the 1700 maybe? But anyway all those 1500,1600,1700 , the older smaller frames are Hydro-syphin systems or whatever its called. they dont have a water pump.
But if the system is clean and operating cleanly and correctly with porper mix of coolant to water it will cool all day long in the southern summer. After all they were designed that way and there is hot places and cold places in Japan as well. These tractors were also sold in Austrailia and Places like Vietnam, both can get hotter than most places here besides texas and FL.
Even a waterpump tractor with a clogged radiator will still overheat.