I think I know why there is no P behind the 400.
Both the Atari 800's were marked as PAL (PAL B/G; for Western-Europe (Germany/Netherlands etc.) while the Atari 400 is marked with a UK tag, which is PAL-I.
That's probably why there is no P behind the 400. I'm pretty sure this 400 was original PAL UK and not modded afterwards. Can't believe someone takes all the effort to make it PAL (with a PAL personality board, and PAL OS ROMS) but then leave it 16K.