Denis was generous and landed me with a NOS cap...but for anybody someday curious, it's pretty clear that the gas cap came from 60's Benelli Barracuda...maybe part number 4122/A. The design language for the steering dampener knob matches the fuel filler cap, later 60's Benelli (metal) caps shows the common design...See the Benelli cap dimension here (not vented)....
PANTERA INTERNATIONAL
A DE TOMASO CAR CLUB
Presents the De Tomaso Forums
The On-Line Meeting Place for De Tomaso Owners and Enthusiasts From Around the World
Clicking on the banner will take you to the sponsor's website.