I remember someone asked a similar question (it was Jeen, it turns out!) a while ago, here on FPN...It was in
in this thread.
In that thread above, FPNer "svejk" posted this:
QUOTE
Hold on! My 200 nibs fit my Pelikan 120s. You have to make sure that it is a real Pelikan 120 and not a Mertz and Krell 120 that was made for but not by Pelikan. I just put a 200 nib in a 120 and it fit fine (and the cap). Then I put in a 250 nib, so now I have another Pelikan 140!
The M100 (recent, not the old 100), M110 and the M150 have a shorter nib and the cap will not fit on these with a 200/250/400 size nib installed, which will thread in, though. So I would think that there is a good chance that a 150 nib will work in a 120.
Try a Richard Binder 200 nib in a Pelikan 120 and you will have what the MC120 italic pen always should have been.
There is info on the two Pelikan 120 types (the earlier one by Pelikan, and the later one made by Mertz & Krell for Pelikan)
here on this page from Werner Ruettiger's excellent Pelikan history site.