Radiator cap for 1935 straight 8

in Hudson 8
My cap is missing it threads into the top of the water level gauge portion of the radiator.
I can't even find a picture of what the cap should look like...let alone find one for sale.
Is it true that these coming systems were not pressurized? I found something in a manual stating the a 4 psi cap was available for cars that frequently drive up hills.
Thanks.
I can't even find a picture of what the cap should look like...let alone find one for sale.
Is it true that these coming systems were not pressurized? I found something in a manual stating the a 4 psi cap was available for cars that frequently drive up hills.
Thanks.
Comments
36 was said to be a semi-pressurised system whatever that means.
Radiator water and gunk get into the fine threads then dry out and glue the cap in place if the car is not used for a while, the cap can be tough to remove even using a spanner. I've learned to take the cap off when I put the car away.