I'm not an expert on either Brough Superior or Harley Davidson, but there are plenty of them around. Yes indeed Harley used a fork with a triple crown from veteran years into the late 1920s, so I agree the fork is probably not Harley. In that case it likely came from the Brough works, because no other bike of the period used a fork like this. The French built a lot of Harley replacement (knock-off!) parts in the mid 1920s, but I doubt anyone at Brough would fit a no-name fork to a 1930-mph racer. I assume the fork was custom-made at Brough for record breaking.
Speaking of knock-offs, the bike in your colour photos at the top of this thread has a different frame, engine, gearbox, magneto, etc. to the bike on which GB is sitting in the b&w photo. Does someone claim it is the same bike that has survived?
Cheers
Leon