Just wanted to know and understand.
Great questions (& attitude) 'luckyrollers' & welcome to the CDN paper forum.
The best, & easiest, place to start is to check this recent thread on signatures
prefixes are 10M large so when there's a new signature then that's presents a change-over signature prefix. New BOC
governors (or deputies) provide these 'opportunities' where one can find the "short-end of the stick" prefix (the one with 2-3M or less) which will be much more collectible than full (10M) prefixes. $10 FME
of the new 2018 series is an example.
has stopped the practice of issuing insert-replacements & will instead create short prefixes since the printers will have an order for X Million number of notes but they will remove several hundred thousand from a number of prefixes leaving the last prefix short. This happened for the 2015 $20 (FWW) & the 2017 $10 (CDF) (for both commemoratives). More info can be found here:https://sites.google.com/view/notap...11-frontiers
And as 'JimmyD' wrote about special serial numbers:
Here are some notes members found out of circulation: http://goccf.com/t/318764
and here is what some members found from bank runs (which may be tough to do from the US): http://goccf.com/t/318765
But I gotta tell you - these notes (even from 7 digit SN) are pretty tough to find so it takes a super amount of patience & persistence.
Good luck & happy hunting!