I have a nice wooden boxed set of BSCY taps and dies, ex-War Department, that I’ve had for at least 50 years, and in amongst the contents are 9/16″ x 20 tpi, left-hand thread taps. I suppose that the BSA M20, the standard fare of the Army despatch riders during the war and into the early 1960s, must have had such a thread somewhere, or they would not have been included in the set ? ( I can only think of pedal cycle crank thread, so perhaps the Army workshops were also fixing pedal cycles ).
