In regard to the Fortresses been given the 'GR' reference.
From what I see on the list of '' only two aircraft have the 'GR' reference. I may be wrong but those two losses(?) were late WW2 .
So maybe it was some new instruction given in regard to Coastal/Maritime Squadron's, if so wouldn't there be a paper trail that could exist to confirm the use of the term?
Or perhaps other aircraft in use or previous use by the Squadron’s have been 'GR' types so naturally office staff would 'normally ' use the term when writing up documents ?
Just putting it out there -
If interested some time ago I made a list of RAF B-17's, references were the AB RAF serial number books, AA100-AZ999,FA100-FZ999, HA100-HZ999, KA100 - KZ999, SA100 - VZ999. 'The British Air Commission & Lend - Lease, and The B-17 Flying Fortress Story' by Roger A,Freeman as well as the now old RAF Bomber Squadron book and the 'Coastal Support and Special Squadron’s and Flying Training and Support Units since 1912.