as i understood it, 704 only saw short flashes of his past life, not all of it, when hwan touched his head. so i don't think he knows eunjae is his brother reborn, nor the fact that he even had a brother before really. he just kinda pieced together from it that his past self and geumsun knew each other/had some form of relationship. the long ass flashback was only for us, as a backstory more or less, to understand the ties and reasonings or whatnot
disclaimer sorry if any of this is wrong!! i did reread the whole thing a couple of weeks ago before the last season started
from my understanding, eunjae doesn’t know they’re brothers in the past life, however 704 does. it’s complicated but parts of 704’s memories were taken when he and hwan were separated during his first life + he got ALL his memories taken when becoming a reaper as his punishment
after they captured hwan in the present, geumsun and 704 read his memories together as part of the deal geum made with yama- where 704 learned of his past with eunjae, hwan, and the king BUT any memory associated with geum were gone. the same thing happened with san where he remembers bits and pieces of his life with geum but hwan stole the memories with eunjae
in summary: 704 knows his entire life story, but has no recollection of his past with geum. this is because hwan stole them and is basically holding them/san’s memories of eunjae hostage in hopes these two lovers would exchange it’s life for those. and eunjae doesn’t know of his past with 704
I don’t think 704 knows his history. At least not fully. His memory is written down in words, not by images - it’s a bit far fetched to automatically assume another person is the reincarnation of your brother. Also it was never shown if 704 gets to read all of them. Geumsun clearly didn’t find what he wanted, so he might just leave and 704, knowing his personality, might decide not to read the record because it’s against the law of the afterlife.
i didn’t think it was that far fetched… ? considering yama made a deal with geumsun only for 704 life records it wasn’t wild to assume it encompassed the details of his punishment + how he wanted most of the responsibility of eunjae’s sins
also i did use a disclaimer!! so i think it might’ve been better to leave your own comment since it’s just your opinion as well
wait does Eunjae know everything that happened in his past life and that 704 was his brother? And when it was said that "the info they were looking for in the reaper records wasn't there" do they mean that only we the audience saw the flashbacks and 704 also still doesn't know about his past life?