I know very little about this derailleur. On the one hand it seems very derivative of a Huret Svelto. The hanger plate is pure Huret, and the placing of the adjustment screws is pure Svelto.
However there are perplexing differences. The gear actually works with a rather nice cam action (rather than a parallelogram being directly pulled out of shape). The pulley cage is pivoted in the middle not at the top pulley as on nearly all early Huret designs.
This story finds an echo in the contention (that is never quite confirmed or refuted) that Huret may have made the first Skitter for SunTour. Looking at this gear it seems perfectly possible that DNB made the first Skitter.
This particular example has a red name plate branded ‘DNB’ (I realise it is hard to see this in a side view!). I have seen similar versions with the red name plate but branded ‘DNB Falcon’.
I have also been emailed by a cyclist with happy memories of a bike they bought new in New York in 1972 which was fitted with a red DNB derailleur - indicating that this model may have been in production for some years.
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