No. 36 MkI filler plug specs

[Zamak]

What are the specs of the No. 36 MKI filler plug? I've seen some in brass and some in what appears to be zinc.
Looking, it appears to be a slightly undersized 9/16-20 BSW thread from the two I measured. Anybody know the actual specs? I'd love to find original drawings? Any other info on inspectors markings would be appreciated.

 

[Snufkin]

Herewith the WWI specs for the No.36 filler plug, and drawings from 1917 and 1974 - the latter included as the 1917 one might not be completely legible.

The main material used in WWII for the filler plug was Mazak (UK) or Zamak (US/Aus/NZ), although brass was also used by NZ and India, and sometimes Bakelite or Bexone by Australia.

The WWI specs are the same for all Mills variants: No.5 MkI, No.23 Mks I-III, and No.36 MkI. Threads are BS Whitworth. There is periodically total garbage encountered that threads were American UNF, but the original specs are what they are.