Note the fact that the spanner has a 5/32nd Whitworth size! I've found this once before on a Triumph wrench:
British Tools and Fasteners has a webpage with a table showing all of the BSW sizes, which include 3/32, 5/32 and 7/32nd sizes. The 5/32nd size is the only one in the series with 32 threads per inch.
Also see Mister G's posting on Herbert Terry & Sons.