I bow to your superior eyesight as it is indeed grey, more obviously so under the liner and it is just dirty on the outside and under the brim which made it look green to my tired out eyes.
Still something of an enigma and after searching images I cannot find any Ambulance marked helmets that look similar to the markings so probably factory guard, though I still have feint hopes that it is HG, though I cannot find any HG helmets marked this way either. The search continues, but the closest I could find was being sold as a factory guard helmet and is shown below.
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