A) I've seen a lot about banknotes from the Philippines with different coloured serial numbers. Normally the serials are printed in black ink but I have some with red ink & have heard (& seen) the odd ones with blue ink.
(post your notes with different coloured serial numbers from this nation)
I've also seen a lot about single letter prefix (the first to roll out & quite uncommon) vs double-letter prefix:
(post your notes with single-letter prefixes from this nation)