In July 1916 Trench Warfare Dept requested approval for the use of cast iron instead of steel for Mills base plugs. In September 1916 TWD met a representative of Messrs Dobson & Barlow to discuss the manufacture of plugs in CI, and both the firms of Dobson & Barlow, Bolton, and Brown & Green, Luton, were invited to submit samples.
The D&B samples were made from machined, malleable CI (basically heat treated white CI) bar, which was not what was required - cast plugs were the requirement. For interest, attached is a scan of the letter from D&B to TWD that accompanied the first machined-from-bar samples.
Brown & Green (for whom Hearn worked) produced 500 samples of a two-winged design, of which some went to James Allen Senior in Twickenham, and were marked with the latter's monogram. Hearn took out a patent on the plug design and the rest is history regarding the No.23 MkII and MkIII base plugs.
John's plug is likely a revised sample version based on the successful B&G submission, before large scale manufacture kicked in. Very quickly other contractors produced the Hearn design plug, and also Brown and Green "executed large orders as sub-contractors for other firms".
Tom