The spark plugs are 14mm...
I bought an Innova Equus #3612 tester and it comes with adapters for 12, 14 and 18 mm.
I don't remember where I got it, but it told me what I wanted to know!
And so why are they metric...? The early manufacturer of the spark plug, Albert Champion, was French.....
The Innova tester has instructions in three languages, including French, so it must be good!