Danny,
Further to your quote from PCS "The markings EZ999 are fictitious and the code NH-Y represents the code carried by an aircraft of 12 Squadron RAAF, in late 1943".
The meaning of this has been twisted. What PCS says in his book, page 172 is:
"The markings EZ999 now carries are fictitious and the code NH-Y represents the code carried by an aircraft of 12 Squadron RAAF, in late 1943".
You can see those deleted two words completely changes the meaning of PCS's statement. 'EZ999' is not fictitious, the markings it now carries are fictitious.