I do own these watches myself. It is just a fact that these chronographs were bought and not produced by GP. It is only in a modern view of things and due to the modern hype for manufactures that it might sound negative. I could make you a long list of other chronographs not actually produced by the brand written on the dial. This was totally normal at the time, for instance Breguet type XX. Just look also at how many brands shares the same "compressor" case from Piquerez, etc.