Why FP5 instead of F6F and AP4C instead of F4U?

[TimeRover]

Why In Enlisted the F6F Hellcat is called FP5 and the F4U Corsair is called AP4C? I have never seen that designations anywhere in the internet, book or game. Where did those designations come from? Are they wrong?

[_WARHEAD]

copyright

The Strange Naming Of US Navy Aircraft

Where does this AP and FP designation come from are they just making sh*t up like the FP-3 is clearly an F4F wildcat is there a reason they aren’t using the actual names and designations?

[Veekay45]

For people without PhD in American WW2 planes, is there a spreadsheet with real names and designations?

AP4C - F4U Corsair
FP-5 - F6F Hellcat
FP-3 - F4F Wildcat
AP-1 - TBF Avenger

Is that correct?
Anything else?

[TheUnclaimedOne]

Grumman will sue DF into oblivion unless they pay to use those names because Grumman SUCKS