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Prior to D-Day there was sufficient concern that allied aircraft would be miss identified and fired upon by shipping that 796 volunteers of the Royal Observers Corp served as Petty Officer (Aircraft Identification) for several months from just before D-Day. They were posted on all US Navy ships and Defensively Equipped Merchant Ships. None were posted to RN ships.
The after action reports indicated that no allied aircraft were fired upon by ships that had Observers on board, however many (cannot find out how many) aircraft were fired upon by RN ships resulting in some at least being damaged.
WI there were enough volunteer observes to be posted on RN ships. Would the RN have taken them?
RN in this case includes British built/designed ships manned by other navies: RCN, RAN, RNZN, Poland, Norway, Netherlands etc.