Vented fuel cap for non-vented gas tank - '72 C20

[540ChevyC20]

Hi All,

My '72 C20 (originally 402 BB) has a non-vented gas tank. I've been trying to find the correct fuel cap for it - it needs to be vented. I've purchased fuel caps both from AutoZone and LMC truck - both look identical - see pic below. Neither seem to be vented as every time I go to fill up with gas there's a loud "whoosh" as I remove the fuel cap. Would somebody be able to point me to the correct, vented fuel cap for a non-vented gas tank? A pic would be hugely appreciated too if you have one. Thanks!