Okay well just to prove that inspite of my bias I do know a little about these pretenders-to-the-crown, the MkXII was the only Griffon engined Spit with a single stage supercharger!
41 Sqn were the first to receive Mk XIIs, then 91 Sqn, a few turned up with 595 Sqn as well.
The major modification to the 21 was her wings. A new design, which had to accomodate longer U/C as well as bigger (5% if I remember correctly) ailerons, based on the piano hinge system and needing weight-tabs to balance them. This was the first ever change in wing shape for a Spit. And because of the taller U/C I believe she was the first to take the bigger (11ft diameter?) 5 bladed rotol prop. Did the XII have the extended wingtips as well Roger?
As for the ace, well that is EASY!!! Ray Harries was the only pilot to become an ace on the Spitfire XII with 9 kills, and was the top-scoring Welsh ace with 16.5 kills total.
Ozzy
Still a few missing there old chap
Most MkXII, but not all, had clipped wings for it's low-level roll. Anyhow, here are all the answers to put you out of your misery 
Griffon Spitfire Trivia
Amongst the production Griffon Spits, what was unique about the MkXII?
A: Well, you could have had the Coffman bulge in the centre of the cowl and the single stage super charger for low-level work, but the answer I was looking for was the MkV style asymmetric Radiator/oil cooler system. All other production Griffon types had the symmetrical layout.
How many front line squadrons operated the MkXII and which ones?
A: There were two, 41 and 91 Squadrons.
By wars end, how many mark XII
