While the ones with the upturned pipes are Norton 16H, it looks like some of the other bikes might not have the dolls head gearbox. If so, that would likely make them the BSA M20 WD version. Also Norton Big 4's are hard to distinguish from the 16H when at the bottom of the ocean.

I asked a couple of the experts about the pipes and one explanation was:

"As a young apprentice, I was told at the time the exhaust was made like that to stop the wind blowing up it and blowing the spark out on the Lucas magneto"

This is from Bob, who is the world's eminent expert on the WD 16H:

"These bikes were made on order of the British Army in India, often referred to as “India Office”.
The pipes were upturned in the attempt to keep the rear end above water when crossing rivers/stream or whatever water.
India office was the only service ordering these exhausts.
They were built between 1937 and 1940. They also sported parts like sump guards, front mudguard grab handles, Vokes Aircleaners, pillion equipment and rearward extended luggage carriers where the British army did not have any of these at that time".