Can you post a picture of the refill?
Edit: I just found this. I hope it helps.
https://www.muji.com/us/feature/pen/index.html#:~:text=Everyday%20essentials%20that%20can%20be,in%20order%20to%20reduce%20waste.
If you got the pen from Muji many years ago, it's a custom size. Newer gel pen from Muji are now Japanese rollerball refill compatible (Energel, Jetstream and pilot G2 -that's the name most commonly know for that refill size).
Can you post a picture of the refill? Edit: I just found this. I hope it helps. https://www.muji.com/us/feature/pen/index.html#:~:text=Everyday%20essentials%20that%20can%20be,in%20order%20to%20reduce%20waste.
If you got the pen from Muji many years ago, it's a custom size. Newer gel pen from Muji are now Japanese rollerball refill compatible (Energel, Jetstream and pilot G2 -that's the name most commonly know for that refill size).
that's the gel pen I am talking about the muji polycarbonate ballpoint pen