Heaters were optional with american cars too. This pretty much ended during the sixties. Most of the US is too cold during winter to do without one ... OK, dress warmly and who needs one? Save $60 on the price of the car, buy a warmer coat with all the money saved.

The coat will keep only one person warm, but the heater warms the whole car & all who ride in it.

Heaters were initially an option in the standard models of Datsun 1000 in Australia but were original fitment in the DeLuxe models. The KB10 was not sold here as a standard model, only as a DeLuxe, so the heater was original equipment in every Auissie KB10.

In the Aussie heat of summer with the heater on it helps keep the engine temperature under control.

Hasn't everyone had a datsun like this at sometime? (gemini even?)

No. I have never had this problem, but then, I look after my cars & I do a full service on the cooling system almost as soon as I get another car. If the radiator does not pass a visual inspection, or a coolant flow test, I have it dismantled for inspection & service, or core replacement.
If the core is in need of replacement, I always get a heavy duty type as it usually cost only a few dollars more.
I always replace all hoses, including the heater hoses, & the fan belt, & if it becomes necessary, the pump & thermostat too.

This way, I never need to turn on the heater in the middle of summer.

Those of us without much money know about the "put the heater on to cool the engine" trick. I used to do that at times, because the radiator in my 1200 was good enough, except for the hottest days of summer. I never once had the engine overheat due to this, but it would get quite warm. I always thought (and still do) that the stock 1200 radiator was just big enough, so when the fins got a bit worn/blocked she'd have troubles.

1000 coupes did come factory with tinted front sceens same tinting as side and rear glass
Also Aus delivered coupes had chrome sill moldings with blue or white strip

The tinted glass was called 'anti sun' glass & i'm surprised that no-one has realised that JDM cars had amber park/ blinker lights in the front apron, while Aussie models had white lenses.

There were not a lot of variations between the two cars [JDM & Aussie], mostly trimmings, but the standard Aussie model was more highly spec'd as many of our cars standard features were available as an option in Japan.
Things like External mirrors, tach, radio, heater, front seat belts.