During WWII Packhard made Merlins under licence and shipped to the UK, each engine had a basic service tool kit containing BSF ring spanners, I have a couple of these manufactured by Blue Point and used them every day for 20 years working on Darts, they are a slender as Snap on, in matt finish, still in the same condition as when they were made 70 odd years ago, still used occasionally as assorted nuts and bolts on my BSA come from redundant AGS stock acquired over the years.