On this picture of ta Tiagra Derailleur on a Dahon Mu D10, there is a second threaded 10mm hole in the frame. Does anyone know what these are for? I have been unable to find explanation.

I can conjure a way to carry a separate derailleur while on tour or perhaps a place to mount the derailleur should the hanger break or get bent into an unusable way. Conceivably it might allow a longer throw derailleur.. Just not clear.

Arguably it would allow a way to "bend the frame" for the Derailleur Hanger to align better but normally you just bend the derailleur hanger itself.

These are also on the Visc D18's that I have. The Mu SL11 I have also has it.

All ears. Thanks.