[Kristorf]

PS: I'm not sure about the yellow diamond either.

Cheers Reflected.

The 'gas mark' was placed on RAF aircraft at the start of the war.
It was a chemical paint that would change colour when it came into contact with noxious fumes.

As for the one on this, I have copied the finished aircraft as painted by the RAF as part of the Battle of Britain Memorial Flight, and who am I to question the experts?