The type of engine fitted and not the span of the wing determined the prefix i.e. LF MkIXc would mean a MkIX fitted with a Merlin 66 engine (low level rated) but the aircraft could have a full or clipped wing. In the case of the suffix letter not meaning the armament but the wing type this is only really true of the C wing as all others had specific armament that related to the suffix letter. A C wing could be armed with either four machine guns and two cannon, four cannon, two cannon only (usually on HF types) or two .50 machine guns and two cannon. This last combination is called the E wing and later models had a specific E wing rather than a C wing armed with E wing armament. The only exceptions to this being in the case of the MkIb that had two cannon only and no machine guns where all other B wing aircraft had two cannon and four machine guns fitted and the MkIIc that was a search and rescue version of the standard MkIIa and retained four of it’s machine guns. The first Spitfire to use numbers rather than Roman numerals was the Mk21 and this began to enter squadron service in January 1945. Post war the MkXVIII was often written Mk18 but the PR XIX that remained in service long after most other types had gone is always written XIX and never as Mk19.
Roger stands back and waits for everyone to tell him off and point out how errors he’s just made.

In the field mods don't count! I mean 92 Sqn actually had 'b' winged models but initially the cannons didn't work, then the flipped 'em on their sides and the damn things worked. So they're listed as flying 'a' wings but actually had 'b' wings. I think this is mentioned briefly in First Light by Geoffery Wellum....

Not nominally 'cos it also refered to the armament as well Woodie. Okay lets take Bader as an example, insisted on flying a Mk Va 'cos he believed in 8 machine guns and didn't like or trust cannons. So we see the case there....
Until the advent of the 'c' wing the armament was what affected the code as the wing internals were almost standard across the board. This is why from the 'c' wing onwards you see all manner of blisters and the like appearing on the wings that weren't on the earlier versions. As we were discussing the Mk V I think you'll find I was right