I've looked at seat medallions before and at that time the bulk consensus was the longer medallions like posted above were on 68's and the smaller ones were the 69's. Although some 69's have the larger ones I'm going out on a limb and guessing they were still using up "old stock".

I also agree with this as my 68 came with it's original rear seat with the long medallions on it. (later verified when I pulled what was left of the B.S. out and matched numbers on it to the car)