They are the gas caps off from 1971-73 Mustang Mach 1's. They can be purchased in eBay. Buy either repros for $90
http://cgi.ebay.com/ebaymotors/1965-...QQcmdZViewItem
or the real oem deal for about $200-250. Just copy and paste the following into the eBay search field to find all the matching items.(1971,71,1972,72,1973,73)+mustang gas cap mach*
Here is a pic of it on my 65 Fastback.