Philippine banknotes [gelöst]

Anyone know if there is some significance between the notes with one prefix letter and two prefix letters? The vast majority seem to all have two letter prefixes. Example I have a P-183 20P 2008A with just one letter (Q618152). All the other notes I've seen of this type all have 2 letters. Is my note special for some reason? Were early or late runs only produced with one letter? It certainly reduces serial number combinations.

Most of the time you start with one:
A 000000000 - A999999999 ---> Z 999999999
when those are all printed you go to double letters AA 000000000 - ad infinitum

So your banknote would be the banknote # 15,618,152 (maybe -16 if no 000000s exist)

"Anyone know if there is some significance between the notes with one prefix letter and two prefix letters?"
Yes, single letter prefixes are the first prefixes issued but I don't believe that the Central Bank employs the letters "I" nor "O" (to avoid confusion between letter & number 1 & 0). Only 1 million notes per prefix I believe (rather than 10,000,000 - 100,000,000 as in some countries prefixes).
Single letter prefixes are rarer (typically make up 5-10% of a Year series) or more scarce than double letter prefixes. So they may command a small premium in BV (Book Value) in UNC.

Thanks for the input. That explains why the single letters are hard to find. I will definitely hold on to the ones I have.