Idk if I remember correctly but I think Mc got the blonde strand when he became kinda like a special rape victim of the brothers? Like the brothers would protect him from other prisoners but he would have to accept to satisfy them, even if he didn't want to.
So I'm assuming that the boy in the flashback and jingi were also constantly sa'd. I think the uncle used the blonde strand has a way to mark them and jingi now does the same with his victims.
It's pretty dark and fucked up

Its been so long so it took me a second to notice and remember what the blonde strand meant. Man this is so depressing