Misplaced my "saved" gas cap during yr+ restoration. Figured another would be a simple find at auto parts stores, not. 60mm neck, with outside tang like radiator cap, not internal like older gas caps. Bought only 60mm stant cap found in lists ( for reservoir), 10bucks, too small (inside of flange measured 55 mm in spots), found some older M-B radiator caps listed at 63mm but big cost just to look.
Cleaning up shop finally found a bag with mine "stored".... Odd looking bit, stamped Ceandens Ltd. Wolverhampton. More web searching, and appears that they used caps from older BSA and Triumph motorcycles... OEM p/n 83-3875. Also appears correct name is Ceandess, Ltd, which may be out of business, as had big tax suit report on google. I found a new one (Ebay) for about 15 bucks + 10 shipping. Ordered one, they had 8. Looked right, hope it fits, will keep as spare, as not something you're going to find on a race weekend.
Probably will be same cap as RT4s used.
Wanted to pass along info for others...
Bob L.