Never seen a seal-spacer-washer like that on any P3 headset. I suspect someone had just used it as a spacer.

Flat lock washers or especially correctly shaped spacers for French bikes were often hard to find (still are) because they used a wide flat inside the opening rather than a smaller "key" to fit into a slot which would fit into a narrower groove found cut vertically through the threads on everyone else's bikes. And a French sized spacer would not fit other bikes with larger diameter threaded steering tubes (25 mm. vs 25.4 mm.). Of course, standard steel keyed washers would also fit a French bike... But, a handy plastic plumbing ring would always provide a simple available expedient if a small shop or an individual lacked a proper spare chrome plated washer ("who'll really notice, anyway... right?").

The top nut on the headset is probably Original... or at least not a "wrong" substitution. "Technically", it was not until the late 1970s that Stronglight had finally abandoned the older P-3 lock nuts designed for pin spanners in favor of the more familiar style with octagonal flats for ALL of their headset models (they take a 32 mm. headset wrench). But, the funky older top-nuts could be easily substituted (and often were) for those from other Stronglight headset models, even during the 1970s.

The 4 pin holes on the earlier style lock nuts were not a great idea. The chrome often chipped after just a few adjustments of the lock-washer after re-greasing a headset. Rust could soon corrode the edges of the holes making not only a messy headset, but one in which spanner pins would not fit snugly and further add to the damage. There was never a specific universal diameter for spanner pins and it was unlikely that a local bike shop would search for one of the perfect diameter for a quirky French headset.

The upper races of both the P-3 and V-4 headsets still retained the toothed top to accept Stronglight's patented toothed lock washers. These worked fine on French bikes especially because those had flats filed onto the rear of the steering tubes to accept the wide flat forged into the lock washer. The knurled sides of the race were easy to turn by hand and so you needed only a single headset wrench. The toothed lock washers held these in place well enough once they were adjusted but they were not intended for a really finicky fit. By 1974 Stronglight introduced their "S-5" ("Super Competition") headset as their top model and these used more conventional flats on BOTH races - probably to enable better "fine tuning" of the bearing tension.

I imagine that 30 years from now people will question authenticity of the now common carbon fiber spacers found wedged into steel headsets of so many older bikes.