I believe these stickers were used about 1969-1970. They have the curved printing pattern on the backing paper like other Plymouth stickers of the 1969-1970 period do. Note the place where the prep person initialed his/her name or initials and date.

I have seen similar stickers for sale on the bay without the spaces to initial in by the prep person. I believe they are from the mid 1970's.

I think the backing paper is a key clue.