Something to keep in mind. Right now in the story:
Jongup is still dating Hana, which means it's before he told Junhong about The Diary. Whether or not Jongup already has the Diary, will be revealed later into the story.

The letters have never been opened before.

That’s what Junhong deduces when sees that they’ve been hot glued with an emblem on the envelope. He had imagined the envelopes to be opened and read. But they are glued shut. Clearly no one has ever opened them.

Either that, or they were opened and read, only to be sealed back again. He can’t conjure up why anyone would do that, but it’s a possibility that he can’t afford to strike off.

He’s boarding the train back to Seoul and there’s a separate suitcase filled with, what he counted as, a total of two hundred and eighty seven letters address to Yoo Youngjae. 

He’s heavily confused and really out of it. The pain of losing his beloved Grandpa, when coupled with the discovery of a secret stash of Letters in his grandfather’s cupboard, is giving him a serious migraine. He can’t wait to just get into a buggy and sleep the headache off.

Waving at his mother with a heavy heart he smiles bittersweetly reminding her to “take care of Grandma, Mom. And call me if anything’s wrong.” When his mother nods with a broken smile, he holds her hands in his own and shakes them repeating himself. “Do call me, Mom. Please, don’t hide anything from me just because you think I’m too busy, okay?”

The woman brings up a hand to caress his face and places a quick kiss to his cheeks as the train’s engine starts to come live. “Remember to eat your vitamins. And don’t worry me and Grandma. I’ll call if something happens.”

“Alright then. Step back a little now” Junhong cautions his mother as the train starts to pick up the pace. He’s at the old train station in their town that still runs