Alan and Joe,
So far this is the only war time M-H tractor I have heard of with this plate fitted.
I understand your theory of steel wheels being used for years by farmers Joe, but I am wondering if this plate was fitted to tractors sent over from the USA under the Lease Lend Scheme for War effort and we know the War Agricultural Committee used a lot of "The Land Army Girls" for farm work whilst their husbands or partners were fighting for their country, so maybe this warning plate was just fitted to those tractors by the War Ag to educate the female driver.
Chris tells me the plate was fitted to the inside of the left hand mudgaurd (fender) when sat on the seat.
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he does excellent work from good digital photographs and measurements, so no need to send your old plate with risk of it being lost in the mail, he ships worldwide.