This has been bugging me
I've just been out in my freezing garage looking at my car.
My thermostat has the bottom opening connected by hose to the radiator, one to the water pump and one to the temperature sensor housing. Keep in mind that my car has twin side draft carbs so there is no coolant going to heat the carb like on a stock motor.
Two possible flow patterns, Thermostat closed and open.
Start at the heater core, return hose goes to the water pump, thermostat closed so radiator is bypased and flow goes to the temperature sensor housing, into the head and back to the heater core.
From the heater core to the water pump, through the open thermostat to the radiator, to the radiator return hose, to the water temperature sensor housing back into the head back to the heater core.
I think this makes sense. For my motor anyway. For example, I had a bum thermostat that wouldn't open. No access to the radiator which resulted in overheating.