The MSFU (Merchant Ship Flying Unit) were eventually allocated the code letters 'NJ', which were displayed in the same form as 'normal' fighter squadrons, for example 'NJ-L'. This code is clearly seen on the fuselage of a Hurricane, in a photo, being loaded aboard, and then on the catapult, of a CAM ship.
These code letters were also used by 207 Sqn, RAF, but as this was a bomber squadron, with Manchesters, followed by Hampdens, and then Lancasters during this period, there would be no confusion with identities.