It's an easter egg on page 6 of the first volume of the SAMTBK Manga series.

First image.

When Claire's going to her new apartment.

The room number is actually the easter egg, believe it or not.

Pull out your phone and open the telephone keypad.

Did you notice?

Let me show you.

The reveal has happened.

Her room is basically Neol.

That's it.

Bye!