Thanks for the responses everyone. The specific problem is that the piston converter for bottle ink seems to be welded into the nib. I'm able to unscrew the barrel and remove the nib/converter assembly, but it doesn't appear that I'll be able to use this pen with an ink cartridge. I'll see if I can take a couple of pics tonight and post them to Flickr of the offending parts.

For a workaround, I went out at lunch today and bought a bottle of Noodler's. It seems to be writing great, but once it runs dry the first time, I'll throw it in the freezer to see if this helps. My plan has been to use bottled ink as much as possible, but I might want to use cartridges on my next trip overseas, as they seem to be easier to take on the plane.