To quote Pelikan's Perch. "The cap was constructed with a metal inner liner, presumably engineered to help prevent cracking".
Thanks for the quote. Maybe I understand something different under "inner liner". As I wrote above, there is something like that extending something like 10 mm into the cap starting from the cap band. I think this is what's meant here.
Matlock and OMASsimo: From the same Pelikan's Perch article: "Over time, the celluloid can shrink ever so slightly around the metal inner liner which can result in hairline cracks."
Shrinking of vintage celluloid is a known issue that occurs with other brands as well.
Rick Propas with the same issue: "I have a drawer full of caps just like that. It is very common and is the result of the celluloid shrinking around the metal cap liner, which, of course, was made to prevent just what it caused."
Bo Bo: Here's an article about the cap click/locking.
I collect celluloid pens and have a significant number of them. The well known shrinking problem seems to depend a lot on the manufacturer. I have not experienced any problems with top tier pens, yet, only with second and third tier manufacturers. Of course, a professional in pen repair and restoration will see all the cases where the issue shows up.
Perhaps the 400NNs made by Merz and Krell in the 1970s did not have metal liners for the caps which could explain the discrepancy. This is just a guess, since I don't have a Merz and Krell pen.
Non of my Pelikans is a Merz & Krell. They are not too hard to distinguish.
All of the caps on my 400/N/NN have that metal insert (about the first 10mm or so inside the cap). Not too sure if the plastic in them is celluloid though... as in, the 400/N/NN Light tortoise and MOP which use the same material in the pen barrel binde and the cap then yes, there is celluloid involved. The black and brown parts in the regular 400/N/NN should be made of more modern injection molded plastics?
Too bad I do not have a sample to test, I know one sure*fire* way to find out if they are celluloid or not...
This is exactly what I think. I'm positive that the 100 and 100N were made of celluloid. I'm not sure if or what parts of the 400NN was made of it. Probably, I don't have enough experience with this pen because I don't own damaged specimens.