Originally Posted by
WizardOfBoz
So is the English threaded cup just for test, or are you planning on using it? That might be hard: remember, the French steerer tube is 25mm OD (outside diameter), and an English thread is made for a 25.4mm OD. So, as a bit of encouragement, you would expect the English cup to be loose! In fact, given the difference in diameter you may have the correct thread.
In any case, without heroic effort you must use a French thread cup on this fork. How does the original cup fit? How well does it fit? So now you have one advantage of that French system, with the little post and the washer with holes. At the very least the cup shouldn't get loose and progressively unscrew.
....
...this, pretty much. If you try the original French threaded race on there and there's a problem, your best bet would be to look around for someone who actually knows about French threading on forks and has the old cutting die to thread one. (In all likelihood, this person will be old.) Then get him (or her) to run it down your fork threads. This will probably be enough that your bike will work OK.
If the original threaded race seems to thread on OK, then you don't have a problem.