It's not really such a mystery. I looked closely at my set yesterday, and if you put them in good light, you can see that they actually do have a hairline joint in the outer face of the hub shell, about dustcap size. I haven't tried to pull them off (I use a cartridge bearing puller for Maillard and Campy pressed steel dustcaps, and it works great), and I'd be very careful not to use too much force and bend them; but I'm betting it can be done. It's really remarkable how invisible the fit is -- says something for French engineering/manufacture.