In the TT you can see that the P-40, P-39 and P-63 are foldered and have an extra variant below them. the thing is I haven’t found any differences between them and the BRs are the same
Just visit the old (and imho way better) wt wiki:
https://old-wiki.warthunder.com/Category:USA_aircraft
P-39 N to Q - better firepower
P-40 E to F - better engine
P-63 A-10 to C-5: Better engine power at medium and high alt
They used to not be foldered so they were grind fodder. But the Snail was feeling merciful (and needed space on the tech tree to hang more).