I have this cover- see attachment, which gives up a great deal of information which I note below, and will I'm sure be corrected by the Army brigade if needs be
The censor mark is CEN CAR SOR (the AR are small and fit in to the curve of the C). It is an RNAS censor mark
The APO is K&C type D1 issued 27.3.17 and No S 41 French Corps Troops. date 18 July(I reckon) 1918
The rectangular censor is type CM6 No 5079, this number having been used in France, Belgium and Italy at this time - I don't understand how, but it looks to be either France or Belgium
The censor has been identified as Lt. Charles S Walker RMA of the Siege Guns
Lastly a red EXAMINED BY BASE CENSOR - are they always red? Any significance?
Michael Gould in his Vol 2 suggests this is 'probably the Carnac Battery' on the Belgian coast. With all the Army stuff on the cover, would anyone confirm this?
I'm trying to keep you all busy in this cold weather
Cheers
Tony