I bought a Pel Toledo with a 18k OB nib two years ago.
At that time I didnt think much about it. Now I wanted to trade my OB nib and while looking at it it looked a little different than my M400 nib. Both nibs are the most recent versions of the Pelikan duotone nibs. But why should the M600 nib screw into the M700 (M400 size) Toledo pen?

Both nibs look almost the same but the nib that I had in the Toledo looks bulkier at the pointy area (This is the shape in general, but not due to the fact that it is an OB nib).

Are M400 and M600 nibs so close to each other in size? I have all Pelikan sizes (M400, M800, M1000) except for the M600, so I need your advice.

I think a picture wouldnt be helpful since the dimensions won"t become obvious
Also, is it true that the special models (such as the regular Toledo) come with an 18K nib. The normal M400 nib is a 14k. What are the M600 nibs made of typically?

I don't know about standard issue 600s, but the Place Series are 18K. So it wouldn't surprise me that a Toledo would be 18K. The 600 does screw into the 400 pen but is slightly larger in other dimensions. It is slightly wider on the nib "shield" area and slightly longer. But at first glance, it is subtle.

The M700 Toledo uses a nib that is the size of an M400 nib, but it's 18K and has a different plating pattern. Here is a photo showing the two styles together; the M700 nib is on the right:



The M600 uses the exact same feed and threaded collar, but the M600's nib is a little longer, and it's set with the feed extended one fin farther out. You can screw an M600 nib into the smaller pen, but assembly tolerances are such that it might crash against the end of the inner cap when you cap the pen.

Thanks Richard, that helps!
Since I got a nib change right away when I bought my Toledo, I don't have the Toledo nib but a standard duotone. It's slightly larger than my M400 nibs. I'll do a fin counting but I assume that the salesperson gave me an M600 nib accidentally.

I'm curious to see if anyone uses a 600 nib in a 400 pen. And if so, did you have any problems with the cap or any other fitting issues?