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I was told orginally these were for the older door strut mounts. It was so you could get to the nut for the mount. The older ones were not riveted.
Jeff is correct - I have an edition 1 with edition 2 upgrades STOL 750 and that hole is for access to the gas strut mounting nut.
I cut an oval hole (actually a slot with rounded ends) near the top and lined it with a cut-to-fit grommet from Spruce for my stainless braided PTFE fuel lines. I also slipped a black corrugated plastic loom over the line to prevent chafing against the inside of the upright if there was any vibration:
I also used the holes as a way to get the fuel lines to the spinny end.