The P3 has a pretty low stack height…about 33 mm. Campy has about 39…same for Stronglight V4 or A9 or Delta, just to name a few others that you might encounter with French threading. Generally, I feel good about a headset stack height if I have a minimum of three full threads of engagement for the locknut. If I am cutting the column, I will measure carefully enough to fill the locknut with as many threads of length as possible without bottoming out. It would be interesting to see how many threads are engaged on the headset in question, but it appears that with the extra washers installed, there *might* be enough fork column for a 39 mm sh headset. As suggested by others, If three threads are currently present, my first move would be to get a proper pin tool to tighten the locknut FIRMLY. Failing that, my second move would be to get a standard Stronglight hex locknut and tighten down. The P3 ( and V4) were good headsets in my opinion….