I just went through this on my 72 challenger. Take the cap off and take a small piece of hose and put it on the the hole you see in the center of the inside of the cap and try to blow through it I bet you can't. Don't try to blow through it with compressed air like I did because it seems like it works with 100 psi but if you had 100 psi in your tank you would probably have a sending unit sticking in your back after the explosion Is it a Stant cap? I think you got one from the same production run that I have. The vent plunger is not drilled all the way through. I took a small drill bit and finished drilling the plunger in the center of the inside if the cap it works fine now

Here is a picture of my gas cap. This is what is on my gas tank that has no vent nipple. It says "Anti Surge" above the tabs and says "Vented" below the tabs.

Is this cap meant to let air in and out?? Or just let air in as the fuel level drops?

Like I said put a small piece of hose on the hole in the center of the inside of the cap and try to blow through it if you can't blow through it it is not vented or it's plugged. In the amount of time it took you to take the picture you could have checked it, drilled and put it back on your car and waxed a couple ricers. Like I said I just went through this 2 weeks ago. You might want to smell your oil you probably have gas in it from all of the fuel that was being pushed passed the needle and seat in the carb flooding the motor

when day is hot or I have shaked the car a lot when driving I'm able to hear a WHISTLE comming out from gas cap ( Stant "vented" ) Whistle comes from rubber seal. I simply moves a little bit the cap and pressure is enough released to quit the whistle.

I have known the "vented" is mainly to allow the air get in as far fuel goes down BUT who cares about air coming in if anyway vapor gas presssure is getting inside ? it won't need get air in with all the vapors.