First of all, what are your stanchions - OEM style or an after market? And stainless or aluminum. There are sooooo many combinations that have evolved over the years that this is a difficult question to answer. If they are OEM style, and SS, they may have a flat just waiting your very sharp drill and then are thick enough for that single post, 5/16-24 thread after market clone. My '31 has a pre drilled and tapped hole on the LH side, but since I have side curtains, ended up with some Sun Spec mirrors, door mounted which work just fine and with virtually no shake, which you will have in the stanchion mounted.

In this photo you might see the hole in the stanchion, though it does have a SS screw in it, as well as the mirror. I have the same arrangement on the pass side, but with a convex mirror which works well too.

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