bark_eater There is an issue that might be there and will seriously affect mounting either a Heuret RD or any other. The hanger type. Older Heuret RDs used a different position for the "B" screw and the hangers reflected it. The hanger stop is something like 45 degrees forward of the common ones. (counter-clockwise looking at the bike from the right side). This was the French standard in the early days of derailleurs and stayed on at least into the early '70s. Older Simplex RDs and dropourts used the same standard. (My '73 Raleigh Competition came with Heuret dropouts and hanger.) The Japanese copied the Campagnolo Campagnolo standard and that became the ISO standard. Heuret and Simplex may well have also moved to the new ISO when it was adopted. I do not know,

So, I am no expert here other than knowing first hand that running an RD of one standard on the other standard of drop doesn't work. Period. Until you modify that dropout. (A French standard one can be modified by brazing or welding in a piece or mechanically attaching it. You might be able to just file an ISO hanger but I'm guessing you will have to add material in the area of the new stop.