[ajmac]

I've got a bit more info.
I collected a 1946 24 stud Flathead for my Loyd a couple of weeks ago and the seller also a had a freshly imported pre 1942 24 stud (from the USA)which I cleaned up to check the original colour in various places. It was a dark green.
A few days later I went to see a very original 21 stud Dagenham lump, it was in a bad way, but on all engine parts it was a light grey. Some other bits I have picked up show a bright green colour. Richard Farrant posted a photo of a British Army rebuilt flathead (1952?) and this had the identical bright green finish.
Perhaps we can err on the side of British 21 Stud wartime units being light grey and the US 24 stud ones being dark green. The Flathead engine shops in the USA sell 'ford dark green' off the shelf, so I imagine they have got it right for the 24 Stud.

The bright green colour comes from the 1950s rebuilds...