I have seen a chain stay braze on mounting which features a ring for the pivot of a derailleur also on an old TAYLOR TANDEM,I do not remember the brand of the derailleur.....only that I passed on buying the complete bike for $125! it may have predated the 2 hole brackets under discussion.Hats off to who ever did the adapter piece as thats what stopped me from buying that tandem.........how I would mount a modern der. a little more enlightened now
BOB FREITAS
MILL VALLEY