As many of you know , the letter following the mark number insofar as Spitfires were concerned denotes the structure of the wing , and how it was ported for weapons and ordinance.
Generaly speaking that's correct, however that's not the case with the IXA and IXB. The comprehensive naming system (such as is used in-game) wasn't formed untill late in 1943 and even after it was many pilots and squadrons continued to use the IXA/IXB designation in their logs and reports.
The Mk IX was introduced using the Merlin 61 then later the Merlin 63 (basically the same thing in terms of performance, just the 63 is of a stronger construction), but then in the spring of 1943 (in March iirc) the Merlin 66 was introduced to the Spitfire Mk IX series. As the 66 gave quite different performance compared to the 61/63 (changes to the supercharger meant the 66 gave its best performance at the same altitudes as the Fw190 had it's best performance) the squadrons needed a way of differentiating between those Mk IXs powered by the 61/63 and those with the 66, so they started calling those with the 61/63 the IXA and those with the 66 the IXB. The problem with this of course is that since the war people have gotten confused and thought that there were Mk IXs with type 'a' and 'b' wings which is not the case, they had either 'c' or 'e' wings.
What does this mean for IL2? Well, any Mk IX with the prefix 'F' is the same thing as the IXA (i.e. powered by the Merlin 61 or 63), while any with the prefix 'LF' is the IXB (i.e. powered by the Merlin 66). For the Dieppe raid you'll want to use the F IXc, 1942 for historical accuracy.
Because I know there are scholars out there on this site who know better than I the exact differences I'll leave it to them for the details , but the broad strokes as I understand them are that the B type wing was faired for one 20mm Hispano cannon and 2 browning .303 machine guns per wing. The C wing had the same weapons ( the Hispano had a different ammo load - I don't know if it was the same mark cannon) , but featured the ability to attach ordinance ( and droptanks?) . The E type wing featured the 20mm Hispano and a single .50 cal browning MG per wing.
Here's a link to a good description of Spitfire wing types:
http://spitfiresite.com/2010/04/concise-guide-to-spitfire-wing-types.htmlIn very basic terms, the type 'c' wing has double the cannon ammo (and a more reliable feed) and has more stable ground handling than the 'b' wing.
Hope this helps,
HB