The short answer is 'RO' (Radio Observer)
but I believe in very early days of AI radar some operators had no brevet and some may have been ex air gunners who re trained,presumably they carried on wearing the gunner badge as long as possible.
Later in the war the 'RO' badge was replaced by the Navigator 'N' badge
Link to RO brevet pic...just scroll down the page
http://www.google.co.uk/url?sa=t&sou...SiNEWrTbmv5UyA
l