I'm not sure how to link to an old post but I found this when I searched on the interwebs...it is actually an fabo post. EDIT: I did a little search and believe it to be a NOX valve which apparently was a CA requirement.
All of this is detailed in the factory service manuals which you can download--up through 73 --for free from MyMopar. Some of them there got there because of the guys on here. For those needing a 74 manual, PM me. It's free