OK, so I just added the air.ini entry from the file, A6M2-AM+F2A_B239_M63--TEST.txt, and now I see the LKk42 gun loadout when I select the newly added M63 plane version. When I saw the "TEST" in the file name, I thought it was some irrelevant file accidentally left in place.
But now I'm a bit in doubt of the new FM being utilized. I've ascertained that the different tachometer response is only due to the code for that gauge as set in the cockpit class, after swapping out my own class file in the testing process. Furthermore, it looks like the LKk42 gun should be using the included MG_BredaSAFAT127 sound files, which are very different from the sounds for the Browning 50. But I'm still hearing the latter sound.
In the past, when installing new planes that had their own FM, I recall that it was required to have the FM file located in the mod's root folder (where classfiles must reside). Is this no longer the case?
Note that I've named this mod folder with a preceding "!!0_", and so it sits in 4th place from the top of the folder listing, with absolutely nothing present in those higher folders which could possibly cause a conflict.