
Spoilers!!
.
.
.
.
.
.
I was right!! What Myeong-il saw inside the letter were photos!
Photos of what? Still not revealed.
(But we've got our theories.)
---------------------------------------------------------
We see a bit of Myeong-il's siblings again. Myeongyoo (sister) has built some resentment against her older brother because the rumors of him being in jail had caused them to be outcasted and ridiculed by others.
During a call with Myeong-il, Myeonghoon (brother) is about to pass the phone to the blind Noona (her name is Nari),
but Myeong-il tells the little brother he doesn't want to speak to the Noona and yells that he doesn't want that woman in their house anymore.
Myeongyoo gets mad at her older brother for insulting the Noona, she tells Myeong-il--
"Oppa, just stay there forever.
Ever since oppa went to jail... people keep saying mean things to us.
And Grandma has been coming home late, because she has to earn money...
Don't even bother to make a phone call!
Just don't call anymore!!"
Myeong-il is taken aback and answers weakly,
"No, listen to me. Don't you know how much I... I want to see you..."
But Myeongyoo stands firm and yells at him,
"I just need Aunt Nari and Grandma!
I hate you so much!!"
The phone is hung up and beeps for a while, but Myeong-il doesn't let go of the phone.
"I don't want to be here either..."
---------------------------------------------------------
Myeong-il and Jingi both come out their phone booths at the same.
And Jingi sees Myeong-il come out with eyes stung red making it clear he had been crying.
Jingi teases Myeong-il "Did you cry?"
And Myeong-il denies it while uselessly trying to jab Jingi in the gut "I didn't cry!!! It's all your fault, you son of a bitch!!!"
Myeong-il walks away, but Jingi still approaches Myeong-il and places a gentle hand on his head and tells him,
"I guess you had a fight with your siblings, but why is it because of me?
You should have done better."
Jingi still teasing Myeong-il.
---------------------------------------------------------
Myeong-il, at labor-duty, thinks to himself that there's a limit to how much he can keep ignoring Jingi.
And that all he has to do to solve it, is to ask Jingi about the letter.
Myeong-il thinks, "What was with those photos...? But I can't ask.
If they're true... Should I ask to move rooms?"
---------------------------------------------------------
Heonjae decides to start holding himself back from kissing Myeong-il in labor duty, and endure being apart while he practices more on the work.
Because Heonjae wants to focus on learning how to do more things, because if he's good at a lot of things, he'll be able to make money to provide for Myeong-il.
He even promises to give Myeong-il all the money he makes, so they can live together once they're out of prison.
Though after making such a resolution to work hard in order to be with Myeong-il,
Heonjae comes to the room and sees Jingi and Myeong-il together at the end of the chapter.

Spoilers!!
.
.
.
.
.
.
-----------------------------------------
Saddest Jingi omg. I've never seen him this depressed before Praying he doesn't start s/h again he looks so depressed because both Myeong-il and Heonjae keep ignoring him no matter how much he calls for them.
Jingi thought Myeong-il and him were okay now, but still continued to avoid him the next day, so he asked Myeong-il "Hey... what did I do wrong...?"
-----------------------------------------
Meanwhile in labor duty, Myeong-il basically gets called a hoe, who only shook his ass to get this job and get special treatment in that room (Jingi's room), but he isn't good at anything and doesn't have any skills for a real job in the outside world.
Then Heonjae was told he had no power here and just hides behind his older brother for everything, the one who actually has authority in the prison.
In short, they were given a reality check that they were nothing without Jingi (the one they're ignoring but is the one providing them with everything they need and want (at the cost of having to obey his Uncle)).
Though obviously I understand why. For Myeong-il's part, he ignores him because of the letter, and on Heonjae's part, it might be out of some selfish possessiveness that wants to gatekeep Myeong-il to himself, but also low-key Heonjae just getting back at his older brother for never telling him anything and always keeping him in the dark, so now it's his turn to not tell him anything.
-----------------------------------------
-----------------------------------------
Damnn, I feel so bad for Jingi (´;ω;`)
He's so... lonely. Especially since he's being outcast by Myeong-il and his own brother. Those two are all that Jingi has here in prison... but even they've left him.
There's this part in chapter 39 where Heonjae is told 'everyone goes through life alone.'
but Heonjae thinks "No, I'm the only one who's alone".
But he doesn't realize that BOTH him and Jingi have felt that they were alone.
Jingi has so much baggage he carries by himself but doesn't tell Heonjae in order to protect him. About the things that happened with the Uncle, about why he couldn't meet Heonjae all these years, about their mother passing away.
Jingi protects Heonjae from all of that, doesn't tell him anything, doesn't share the family problems, or the burdens as the older sibling.
And there's nothing more lonely than facing everything on your own.
In a way, Myeong-il is lonely in that sense too. Though he has his family with his grandmother and siblings, having to carry the responsibilities of being a parent and older brother to his siblings, and a provider of the household, all while he was still just a middle school student and having neither of his parents to turn to. It must've also been pretty lonely having only himself to rely on.
So now, here these three helpless people are... possibly co-depending on one another in the future (hopefully).

Oh, I read here in new toki though they're only raws and not yet translated: https://newtoki468.com/webtoon/46802614?toon=BL/GL

ALL of this yes. Never thought i would genuinely feel bad for jingi. Particularly the relationship between the brothers makes me feel conflicted like the way jingi just takes all the responsibilities without getting anything out of it to heonjae who's kind of in his own world and sorrow without realising that he's depending on him to leaving his brother alone for bussy is just idk it makes me mad because they've only known myeongil for maybe two months and it's crazy to me how even after 10 years of close enclosure with jingi he still is ignorant to what is happening. At the end of the day that's your brother and that man has been taking care of you for years helping you get out sooner helping you be safe, the moment he found you again he tried his best even in his own fucked up way right so how could you just throw him away hahhhh can't believe I'm here defending jingi lmao what is this webtoon istg

Yeahh, it's sad Heonjae doesn’t really see what Jingi's been doing for him. I think it's the result of how Jingi, as the older brother, overindulges and overprotects Heonjae from a lot of the serious problems that Heonjae needs to face by himself.
But since Heonjae is sheltered by Jingi from all that, Heonjae isn't able to see just how much Jingi has done for him.
Myeong-il once said that it's actually Heonjae who is skilled and can do things while Jingi does nothing but order people around, but neither Heonjae nor Myeong-il realizes how much they're actually depending on Jingi.

Nicee, I'm glad more people are reading Slammer Dogs despite some of the disturbing themes, and are pushing through those just to read the progression of the story.
Because I genuinely enjoy this. It's so unique, I don't know any other BL or manhwas like this. None of them look like this and none of them tells a story like this.
It reminds me a lot like a comic book, with the way the panels are framed, the way the storytelling is conveyed visually, and all these small details and clues that we get to piece together as the chapters go on.
So, I'm glad to have more people like you onboard this story. Hope to see you until the very last chapter together! I need to know how this ends.

This is a pretty interesting take!
I think the reason Jaeju said that when he was younger is because he WAS the only lonely one then. Jingi at that time was apart of some illegal company, and presumably, did have more friends.
I personally don’t feel bad for Jingi at all. Jingi is a criminal (they all are), he’s being isolated because of that letter, and also..well, he is a grap1st. That letter is so bad myeong-il wanted to throw up. That letter must’ve been horrible. He might be lonely, but as you said, they are all lonely in their own ways, and i think that the three of them deserve it.
:// hmph. MLs like this don't really have a satisfying character development or redemption arc from their sh*ttiness. Especially with weak looking FLs.
He'll probably do something really really sh*tty and it'll be revealed he only dated her as a bet.
FL will be hurt and resentful towards him but only for 0.5 seconds before the ML saves her from a random situation (from her ex, or other guys, or getting hit by a truck) and then the ML gets hurt saving her, so FL ends up quickly forgiving and falling for him again.
The end.