Thread form is actually an obscure thread being 7/16" BSB.
BSB is British Standard Brass which is a 26 tpi Whitworth thread form (55 degree thread angle) as opposed to the 60 degree thread angle found on BSC/CEI threads.
As luck would have it I am making a batch of these nuts tomorrow night as they also fit the side lights on my 1955 Plus 4 DHC.
So rather than pay the frankly ridiculous sum of £8.20, I'll make you one and post it to you!
PM me your address and I will get one out to you.
Arwyn
