Fuselage code is WD, which would be the 335th Fighter Squadron of the 4th Fighter Group, which was originally an Eagle squadron, so it could very well be a squadron trainer. The Eagle Squadron did fly Spitfires.
Strictly speaking, the 4th FG was not a former Eagle Squadron. It was
made up of three former Eagle Squadrons... 71, 121, and 133. When these three RAF squadrons were inducted into the USAAF, they became the 334th, 335th, and 336th Fighter Squadrons, respectively. Although the codes 'QP', 'WD', and 'VF' were used by the 4th FG for most of the war, they were also used by the 52nd FG... 2nd FS (QP), 4th FS (WD), and 5th FS (VF).
There was only one other such instance of duplicate codes within the USAAF... the 78th FG and 31st FG both used 'MX', 'HL', and 'WZ'.
While the 4th FG did fly Spits early on, (from September '42 to April '43) the codes used were those of the former RAF units... the 334th was coded with 71 Squadron's 'XR', the 335th with 121's 'AV', and the 336th with 133's 'MD'. To the best of my recollection, 'QP', 'WD', and 'VF' were not applied to the 4th's Spits... but I could be mistaken on that point.
In any event, as all the 2-seaters most of us are familiar with were post-war conversions, there were obviously no 2-seat Spits in use by the 4th FG. (or the 52nd FG, for that matter) During the war, I believe there was at least one rather crude RAF field-mod to make a 2-seater, and the Russians also made some conversions. But I think that's pretty much it for wartime 2-pit-Spits.
Back to the airframe in question...
Why these markings were chosen for a 2-seater is anyone's guess, and the same question can be asked of the Collings Foundation's 2-seat P-51C. But I guess it all comes down to the fact that the guy who owns the bird can color the feathers any way he wants.
Anyhoo, here's another view of this Spit while still in USAAF colors.
Harry Stenger photo
And the original ship after which it was marked...
Image credit: unknown
From what I understand, it was a bit of a job to balance the markings for a Mk.IX two-seater as opposed to the original Mk.V. Here's a look at some of the initial layout work, using ML407 at Duxford as a 'canvas'...
Image credit: unknown
And here's
what this Spit looks like today...