Grade 316/A4 Stainless steel Dome Nut . (Also known as “Acorn” nuts ) 1/4 BSF  (BRITISH STANDARD   Fine) thread, 

These nuts are  NOT polished although they will polish superbly if a brighter finish  is required .
Difficult to obtain this pre war thread and in this top grade of stainless steel , especially in small quantities 
Surplus New/old stock from a Fastener Company (Stainlessparts) in perfect condition. Pack of  6
An exceptional bargain !