It's just a catch can. If the car has a 160 stat and proper cooling nothing should be in the bottle.
Cars in the 60's didn't come with an overflow. They just filled to 1/2 inch below cap. Example a 1967 Camaro
The reason for one is tracks require one to be at least 16 ounces. So in case a little water does spit out, it doesn't do it on the track surface.
The billet ones used on street rods are neat.