I have to suppose that the frame was designed for smaller chainrings (or that the builder simply wasn't thinking).
In any case, you only options are a low band, of bottom bracket mount derailleur.
A typical braze-on adapter wouldn't work because the band is close to the height of a standard clamp-on band. In they make a low band braze-on adapter, that would be ideal, since it would allow the full selection of derailleur options.
The other option is to file out th lower boss, which is useless anyway. Or if there's enough meat on the band, file a clearance for the boss into the band.
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