What year are we talking about?
I think they are referred to as a "coin". I believe the number represents the last two digits of the part #. They stopped using them at some point.(73 I think) My 73 doesn't have one. I have a K-member out of a 72 Demon and it DOES have a coin. The B-bodies went to a different suspension setup in 73 so my guess is a coin wasn't needed anymore. The E-bodies all took the same part.