Eddie, the bolt hole is a modified lamp boss usually seen on front forks. If
Hetchins went to the trouble to modify it i.e, the sides have been filed to
make parallel ''flats'' , they may well have made a bracket to carry the
derailleur. In this case it could be any of the popular mechs of the time.
Huret also made a cam operated front mech that could fit. You did say that
the frame has brazeons for bar-cons, so this rules out a rod type front
mech. I have a Bill Liddell frame where a lamp boss has been brazed to the
down tube to carry a water bottle cage. It also has ''flats'' on the side so
I assume that a special cage was made to fit, or more likely a proprietary
one modified. These sort of 'mods' were not common and certainly adds to
the 'Mojo' of the bikes so treated, don't you think? The French
'constructeurs' were the kings of this practise as lovingly drawn by Daniel
Rebour.
Mark Stevens
Evanton Scotland