Pilot MR Metropolitan/MR Animal collection/MR Retro Pop collection nib replacement
If you are interested in replacing the M stock nib on a Pilot Metropolitan (US version) or Pilot MR (EU version) fountain pen, there are no nibs sold by the company.
The good news is that Plumix, Pluminix, Metropolitan, Prera, Penmanship, and 78G fountain pens share the same pen and they are interchangeable.
For a EF nib you may spend around $10 on the Penmanship fountain pen and replace your stock M nib with the EF nib.
Good luck and Pilot them out!