There's always going to be one or two corner cases, whether import bikes, higher-end bikes, otherwise, which doesn't fit the mold. Maybe produced for UK market, for example.

On north american sourced bikes, Peugeot absolutely started the French/Swiss conversion in '79-'80. They started phasing out Swiss for BSA in '83-84, you'll catch oddballs around the peripherals of those four years which are either/or. The head lugs on the bike shown and the other general features look typical of a '80ish low-end model. If it's a north american bike and the cups are threaded opposite directions, I'd buy anyone here a beer if it's actually BSA.

I'll just re-write this last paragraph so nobody thinks I'm jabbing at them. That's not my style.

Swiss is close enough to identical w/British - same thread direction, thread pitch is close enough I've picked up more than a couple frames re-threaded by the PO and the threads are jacked, nothing stays snug in them. It's an easy mistake to make, you can get a good half to full thread (or more) with a British cup on the lower-end Swiss-thread bikes. These cheap Peugeot bikes sold like hotcakes in my local market since the early 2000s and made up half of my flips to fund parts and bucket list bikes, that trend finally ebbed last year. I've had more than my fair share of experience on this one. These "sport" carbolite frames are what they are...

PS - I've restored, torn down and/or overhauled no less than two dozen PX10s in my lifetime. I don't recall ever finding one that was Swiss-thread. Only French or British. Now that I think of it, I don't recall any of the higher-end pugs Swiss-threaded. No doubt some exist somewhere... If I did, I may be suspicious if it weren't just a PR10 with a PX10 fork applied.