Hey man, so in the new
NEEDED INFORMATION section you asked for Wii U serial numbers information and Splatoon box information .Just recently i got both of these
My wii u's serial number (which is the plain wii u basic edition with no games bundled with it at all (white)) is GEH10014410 and it arrived with firmware 4.1.0 (or was it 4.0.1? I really can't remember xD)
My Splatoon box has this in the back (which i believe is what you want) WUP P AGMP and right below that there is TSA-WUP-AGMP-EUR i used it to update to 5.3.2
Hope i helped