Same nib I believe as the Graduate. It's a #5 nib with a #6 base diameter. It's been a question asked many times. I'm not sure I have seen a good answer yet.
I've seen a proposal to take a #5 nib and flatten out the curvature where it goes into the pen. I've not tried it myself. I would love to see a working solution though.
I use my Vertex S. FP daily and couldn't find a higher quality nib that was compatible; they kept sliding out. I am going to bring it to the pen show and have one of the nib guys take a look.