[Skunkmeister]

They dont give a date in the book. They say that the first operational squadron to recieve Mk.VI's was 418 squadron, in Spring 1943. I'll dig up some other books to see what I find.

EDIT: found a date. 418 Squadron first recieved their FB.VI's on 11 May, 1943. This is from Squadron-Signal publications. They also state that the RP rails became a standard option after the trials.