Thanks, Danny42C ... this Thread is going to be a fascinating read.
I have nothing of substance to contribute, of course! However, it therefore seems [if it read it correctly] that EZ999 is EZ999, with faux markings, and may have been 'tinkered with' during its time as a training airframe. On that basis, a replacement rear gun fit and new [or no] sqn markings would make the venerable bird 'kosher'.