.303s instead of .50s.
The bottom one is a Spitfire LF.Mk IXe, so no .303s on it, the two aircraft are visually identical. Spitfire LF.Mk IXe entered service in early 1944. Spitfire LF.Mk IX with the "universal" wing (two .303s, one 20mm cannon per wing and no underwing bombs) went into service in early 1943, with the same engine as the LF.Mk IXe In AH our Mk XVI is actually an LF.Mk IXe as its full throttle height is that of a Merlin 66, not a Merlin 266.
TRASH99,
Most of us don't use searches on the internet for data, but rather books from our personal libraries. You are revealing your ignorance about the subject quite dramatically. Guppy knows what he is talking about in regards to the P-51B and P-51C. The only function of having one of each would be to have one with the Malcolm hood, as our P-51B does in the game now, and one without it.
As to what Ack-Ack is saying about the Spitfire Mk XVI that you seem to be missing is that the 23 hours of combat time you are quoting was for a single example of the 1053 Spitfire Mk XVI's that were built. A great number of other Spitfire Mk XVI's also saw combat and the total combat time for Spitfire Mk XVIs would be in the tens of thousands of hours.
If you want to read over a good site for WWII aircraft information, try:
http://www.spitfireperformance.com/Contrary to its name, it has primary source documentation for many different WWII aircraft.