As others have pointed out, the split is from the stem being installed with too little insertion. French steerers are thinner than BSC or Italian by 0.1 mm which doesn't sound like much, but it's already a thin, threaded part. Most of the French forks I have seen only have the flat near the top, but they seemed to have gotten carried away at the Gitane factory that day. Still, it would not have been a problem if the stem had been correctly installed. Practically speaking, this is not a highly stressed area under normal use, as the stem provides significant reinforcement. I would not do this for a customer, but if it were mine I would insert a French stem, tap the bulge out with a hammer, put it together with the stem inserted properly, and ride it without any fear. Even in the unlikely event that a crack would propagate around the thread, which I believe is highly unlikely, a failure would not cause the loss of control one would get should the steerer separate below the stem. Replacing a steerer takes a lot of heat, or a mill and some expensive time. It is not a trivial task.