[RichardJ]

From the Stant web page;

What is a "pre-release" or Pre-Vent fuel cap?

Some Stant fuel caps have a "Pre-Vent" feature. These caps are designed to allow the controlled release of any pressure that may have built up in the fuel tank, so the tank is de-pressurized before the sealing gasket is lifted from the filler neck. This dramatically reduces the possibility of any fuel expulsion from the tank to your hands or clothing.

Are fuel caps for today's cars vented?

Fuel caps have not been fully vented since the 1960's. Modern fuel caps typically have valves that relieve positive and/or negative pressure once it reaches a certain level.



I interpret that to mean that while the cap is not vented, their Pre-Vent feature, will relieve any negative pressure that will cause a problem with these old carbed non-evap systems.
I have what I think may be the same cap, but it has been on there so long I can't find any name or number. I has worked fine with no issues.

Summit Racing lists the Stant 10491 as fitting Chevy/GMC truck from 1947 - 1990