If Hurri600 says there’s a picture of V6799 as SD-X there should be no further doubt. I haven’t got the mentioned book, but take his word for it.

As I said in my original post (top) the replica at Gloucestershire’s JetAge Museum is finished as SD-X and is endorsed with Mackenzie’s signature; the code SD-X is also confirmed by J.D. Rawling’s Fighter Squadrons of the RAF book (p.457) 1969 edition.

RJC