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Sure one of us can dig out a flat style and post it to show the earlier style for comparison. Didn't have the time today since those are all put away in round tubes
Until then the arms in your reply #8 are the earlier 65-mid 68 flat clip style and the ones in reply #11 are the mid 68-up curved clip style.
The originals may be worth restoring 50 years in the future when there is no alternative and then it will be prohibitively expensive unless money is no object. There is a lot of labor to disassemble the arm ,take the diecast hub flange off ,repair the pits , re satin chrome plate the finish (special plating) and reassemble. There are still many out there in good condition to find. I have a few excellent pairs to sell in case you cant find any . Remember millions of Mustangs (66-mid 68 ) used them. That means a lot are still out there. The trick is knowing where to look.