Also worth noting all those 'secrets' can be done with Bonuses.
a - If you press "N", and nothing shows up. Tap it again, because it sometimes goes to "neutral" by default :)
Bonus -> First hit turns into neutral, second hit turns into N.
If (Slot Letter) is alphabetically before N: Turns into N.
If (Slot Letter) is alphabetically after N: Turns into neutral.
Also, hint: Leave slots AA-AX for smaller nations.
Something similar happens with A:
Bonus -> Sets back to A.
If (Slot Letter) is A, Z or any letter inbetween, turns into AA.
If (Slot Letter) is a double-digit slot, skips to the next slot (AA -> AB, AB -> AC, AM -> AN)