There are only so many ways to design a Pilot's Chronograph so all are derivative.
The question then becomes who has precedence or prominence in the Pilot's watch category?
IWC is pretty well placed for Pilot's watches to the extent that during WW-II, they supplied air forces on both sides of the conflict - RAF and Luftwaffe. I don't know when Fortis came into the game.
Regards,
MTF