thats play whats is called and where do i get one

Here is another view of the part that Alaskan T/A shown you. May be this will show you, what I think you mean by the ring, a little better.

http://cgi.ebay.com/ebaymotors/Mopar-71-74-Duster-Dart-filler-tube-trunk-seal_W0QQcmdZViewItemQQcategoryZ42604QQihZ013QQitemZ230178823204QQrdZ1QQsspagenameZWD1V

Or is this still not what you are talking about?