Whipstrut.... There is nothing special about these items. Unless you are absolutely determined to use genuine parts, either used or new old stock, they can be produced locally. Luckily for you, searching the first item in the Literature Section above will bring up a useful table of part numbers and specification. Joolstacho.. Nut and Bolt.
The first post is by the highly regarded Muskrat, and following the link to stainless should give you the information you need. The alternative is readily available so called "Pattern Parts", of varying quality, and a choice of material, mild steel or stainless. If you make your own, any readily available fine thread would do for the fine end. BSA used the BS CEI (Cycle Thread). As their heritage was in bicycle manufacture, this thread was also most suited to the motorcycle manufacturing requirement. BSW is the Whitworth thread, the coarse thread used where the studs go into the softer aluminium of the crankcases. So here you have to stick with Whitworth.
Swarfy.
