'71 Dart Fuel Cap ?s

Looking through the Classic Industries catalog I've found replacement gas caps.

They list:

MA8173 - 1967-71 A-BODY FUEL CAP NON VENTED-PRIMER
Reproduction fuel cap for 1967-71 Dodge and Plymouth A-Body models. Manufactured as original, this non-vented cap comes primered and ready to paint and install. ($29.99)

MF368 - 1967-76 DODGE A-BODY GAS CAP
Reproduction of the fuel cap for 1967-76 Dodge A-Body models including Dart, Dart Sport and Demon models. Manufactured as original with a primer coating ready to paint to any color. This is a vented cap. ($39.99)

How do I tell if my car needs the "vented" or "non-vented" gas cap?

Thanks for your advice/guidance