Used vented fuel, or gas cap as installed on 1967-69 Mercury Cougar and 1970 cars without California emissions. Also fits 1969-70 Shelby Mustang. In 1970 California required a non-vented fuel cap. This version does have a check valve, one way vented. This is probably a service replacement fuel cap. Typically, OEM factory caps only said "VENTED", see our related item for that specific gas cap.
We stock these in limited quantities, and will have an intact paper gasket. "ANTI SURGE" and "VENTED" embossed and has the "S" stamped onto the center rivet.