I was reading some back posts and saw that the replacement ranges were reported by members who are/were "brickers" and that some are not doing this anymore. Firstly is this correct, and if so, is it possible that some replacement ranges were not catalogued, which could explain large gaps in missing SNs for some of the later prefixes?
It wasn't just "brickers" who reported replacements discovered in other bricks of notes (or different prefixes where they should not have been). All of us reported discoveries & they were reported to Gilles P who kept track of it all and that's how the insert replacements were documented.
The EGR $20 occurred in 2008 & the EJB prefix was printed in 2011 so I don't think they're related. The printers have never been committed to full 10M runs (they may try but there will always be some that are shortened or have gaps).
"Could there have been replacement ranges not catalogued?" I'm sure there could have been a few ranges- especially
SNR ranges as these were tiny (200 to 2000)- that could have been missed. The system wasn't infallible. I think we covered this possibility in older posts here years ago but I don't think that accounts for the gaps that have been (& not been) noted with the Journey or this (& later) Polymer series. "Brickers" & collectors both noticed that there were less and less replacements showing up before & around 2013 (the intro of 2013 polymer saw many ranges & then the inserts/SNR just dropped off).
They noticed big SN gaps in the 2015 Commemorative $20 & it was then that Charlton formerly announced that insert replacements were no longer a policy. Instead extra notes were be printed for an extended prefix (FWW). The first FIVES & TENS were introduced in 2013 & the policy (not to include insert replacements) could have been introduced after the first 10 prefixes. The FTG is the last sheet & SNR with smaller & smaller ranges. FTH $10, BSW $20, AMK $50 & EKZ $100 all could have been extended prefixes used as replacement notes to reach 10M runs for other prefixes, but we'll never know for sure since the majority were discovered well circulated (we just know they're scarce!) We just know these prefixes were super short/limited.