What is found is, editing the text POKE RANGER B5BJ01- to POKE RANGER B5BJ00- for some odd reason breaks Japanese redemption. Editing the 01 to anything else breaks Japanese redemption
I wonder if that's related to language recognition.
edit: It's probably just game cart recognition. NDS games have similar title structure too. Something something 01
Also, removing the AGB header breaks redemption.
Interesting, changing all bytes from range: 0xC0 to 0xFFFFF, to data: 0x01 or 0x0100 breaks redemption too.
Filling them with 0x01000000 re-allowed redemption.
(0x00 made it work fine, as described earlier by BlackShark)
edit2: redocumenting decchi and PCD relations
it still works when it's trimmed down: final byte being 0x1004A7
Heck, it still works when 0x10010 to 0x100FF are replaced by 0x00s
edit3: What if we're misunderstanding how the block works.
Maybe it's not looking for a "region code", but rather, the field where extra cards for languages could have been are not populated, hence it can't find a card? (probably not...)
Given that I blanked out all the apparently unused areas, and trimmed down the size, I'm guessing it's the NDS cart that is searching for a particular value within the 'useless' region. (Given that changing that entire sector to non-00 kills the distribution for JP)