Predominately you see the rounded rivet on the 68 lower arms as well. The majority of the 68 arms I did had the smooth domed rivet. I know there was a transition at some time late 68 because I have yet to see a 69 original lower arm with the smooth dome. Have only seen the waffle pattern on 69 arms. If you look closely there is a difference in the 65/66 smooth dome versus the 67/68 smooth dome as well. The jack tabs on 67/70 arms also vary a little. Some have the small tit sticking out & some have a less predominant wider section sticking out is the best I can explain it. Seems like the skinny tit is more 67 from what I remember, Just my observations.