I will go on a limb here. The decals were available from your Plymouth Dealer but were not affixed to cars as the "Scat Pack" Decals were. I believe 1971 is when they added the "Member" portion of the decal. I say this from examples from the catalogues of the day.
The first three photos are the 1970 catalogue and 1970 RTS Banner
The last two photos are from the 1970 catalogue
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