John, only the dimple (or other marking) in the center of the rivet head is significant, denoting the rivet material. The other markings are manufacturers marks. As you know, the single dimple indicates an "AD" (or 2117 aluminum) rivet (and doesn't that dimple make a great starting place for drilling the suckers out?). If you see a rivet with two dashes on the head, it's a "DD" (2024). A raised dot is a "D" (2017), a raised cross is a "B" (5056), and no mark is, as pointed out previously, an "A" (soft 1100 aluminum; used in fiberglass, to attach plastic wire tie-downs to frame members, and other non-structural applications). I know... WAY more than you ever wanted to know!
Ben R.
RV-9A #90217
First wing almost done
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