The early Datsun Roadster had a radiator setup without a cap on the radiator itself, there is a cap on the thermostat housing which was lower than the radiator, but it was just a cap, it had no pressure relief, it was a pain to get the air out of the system.
The radiator did have a small hose in the top middle that went to a reservoir that did have a pressure relief, but it also was lower than the top of the radiator.
As I said, it was a pain to get the air out of the system, when I upgraded to the 1968 roaster model drive train, I used that radiator because it had a traditional radiator cap on the radiator, no more issues getting the air out of the system anymore after that.
The problem with that early system was that one had to drive the car till it was fully warmed up with the radiator only 2/3rds full, and then let it cool off, when it cooled it would pull the water from the reservoir into the radiator, then you had to fill the reservoir up, start it and fully warm it up again until you quit getting air bubbles in the reservoir, then then shut it down again and let it cool down so it would pull more coolant into the top of the radiator thru that little hose, you have to keep doing that till the reservoir fluid quits dropping, then the system is full, WHAT A PAIN!!!
I was sure happy when I seen that 1968 radiator with a cap, you can see it is not a pressure relief cap, it's just a cap, the pressure relief cap is still on the reservoir that has that hose you can see connected in the top middle of the radiator, you can also see the reservoir and pressure relief cap itself on the left where that hose leads to.
“The difference between genius and stupidly is that genius has its limits” Albert Einstein