The RAF used a clockwork IFF device during the BoB which was codenamed 'Pipsqueak'. Info on the web including some interesting transcripts of messages.
Given the low level nature of allied aircraft operations at D-Day and the almost indifferent attitude to aircraft recognition by ground and surface vessel personnel, such identification markings must have prevented many instances of friendly fire from those who preferred to shoot first and ID later. Navy gunners were particularly trigger-happy. It was an unwise pilot who approached ships in a manner which could be construed as suspicious or potentially aggressive.
Despite those stripes, there must have been many losses from friendly fire - above or below.
'Find your enemy and shoot him down - everything else is unimportant.'
