Thanks Trev,
Will have a better look after, but the bit that I do suddenly understand, is, why it's 'a closely guarded secret!'!
Sounds like maybe a primitive kind of relay??
Had a look at that John, and I say yup, the flywheel is removed, but looks like maybe(?) one of the ones with 6 side bolts as no bolt-hole above the journal where one holds the sludge trap, a bit odd tho'...?