I don't know what the hole in the down tube, but some frames with a similar web between the chainstays have holes for FD cable routing. I'd mount a crank and front derailleur and see id the travel and angles are OK, and if so, simply use a bit of body filler above and below the braze-on for looks. Otherwise, drill out the rivets, fill the tube back to contour, and use pop rivets to affix the braze on.
Nice score. Enjoy it.
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FB
Chain-L site
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