They changed every year until late 69. In the summer of 69 the final design for the 70 was completed. You will also find lower core supports with tabs for the upcoming Duster valance, I have seen quite a few of them on original cars. The 67 is completely different from the 68, then in 69 in changed again, then later in 69 like I said above. I have done extensive research on these 3 years but still have no idea why it changed so much. Also the 68 and 69 GTS hoods have different underhood bracing with a couple of crumple zones in the 69's.