71 Demon 340 Gas Cap??

Try these links these look like what you need:

http://cgi.ebay.com/ebaymotors/Mopar-70-71-72-73-74-Duster-Gas-Fuel-cap-Primer-NEW-_W0QQcmdZViewItemQQfitsZModelQ3aDusterQQhashZitem5d2262157dQQitemZ400008811901QQptZVintageQ5fCarQ5fTruckQ5fPartsQ5fAccessories#ht_500wt_896

http://cgi.ebay.com/ebaymotors/67-76-Valiant-DART-Duster-DEMON-MoPar-GAS-CAP-68-69-70-_W0QQcmdZViewItemQQhashZitem53e7ea65dcQQitemZ360373183964QQptZVintageQ5fCarQ5fTruckQ5fPartsQ5fAccessories#ht_500wt_1182