The IA was the first production version, and went into service with two blade prop and "straight" (not bulged) hood.  I believe that the first scheme was the green/brown camouflage, blue/red (type B) roundels on 6 positions.  Several squadrons were equipped with this version, so it's hard to say which s/n can be considered the "original".

I'm sure Ozzy will add to this.

Is there any particular s/n you're looking for?

There are CFS2 versions of the MkI available both here and at flightsim.com (different models) that work in 2k2 and FS9.
K5054 is currently only available as payware
The last freeware version of her was in FS98, so come on designers, make one for me please.

Ozzy

Squall12990 and Ozzy72 are in luck.  I'm currently making Spitfire I in gmax. 

Don't expect it anytime soon though, I'm not very good at modeling yet.

If I could find a set of specs, a few colored photos of the plane exterior, and picture of the panel and cocpit, weapon stats and aircraft stats of the first mk 1a than I would have a cute little project on my hands.