I recently purchased a Mentmore Diploma from the late 40's or early 50's, and discovered that the slip cap doesn't have an inner cap. The first symptom was that there didn't seem to be any firm stopping point for the slip cap when I put it on. The second symptom was that the nib was drying out pretty fast. The final symptom was when I made sure to push the cap down on the pen to prevent drying, and managed to bend one of the tines of the nib against the inside bottom of the cap. I've looked inside (now, I didn't think of it earlier) with a penlight and I can't tell if there ever was an inner cap.
I presume or at least hope I can get the nib fixed (it was a snappy little medium italic). But there's no point in doing that if I can't secure the nib from drying out all the time. Should I have expected this slip cap to have some kind of inner cap? And is there a way to put some kind of replacement inner cap in? Is this (the inner cap, as well as the nib) something that an experienced repair person would be able to fix? Or is the pen just a dead loss?