Already sent this directly, but here's the end-all-be-all of info you'd want on the Stronglight P3, V4 and S5 headsets:
Stronglight headseta
The V4 has V-cut grooves for the races, if I recall. Easiest way to distinguish between it and what the P3 uses, and to tell if you have a mis-matched headset. I find the V-shaped races lend to a smoother experience, but that's entirely IMO.
That said, the cups here are press-in, no threads needed, just like many other headsets. It's not uncommon to find ridges like what you're seeing in the head tube, the top and bottom cups are still press-in. The only two threaded pieces in the headset are the top nut and adjustable race with "alligator teeth" which lock into the space under the top nut.
The stack you show in the 4th post above looks to be complete V4 headset (judging by the top nut) assuming there's a crown race under the bottom cup. I have no idea what that is in your 1st post, I've never seen that top nut before.