Need to call and make sure this one is plastic, but I'm pretty sure you want one of these:
http://www.bugcity.com/cgi-bin/WebSt...chcd=311941177
I believe they went to the plastic rings in 1977. That should help.
UPDATED:
You may want to make it easier and go right to:
http://www.finishlineaccessories.com...roducts_id=186