Posted 19 January 2019 - 22:00
I'm a bit pressed for time or I'd do the heavy lifting. It is a common subject when speaking about true vintage pens (going back on the eariy part of the last century). Simply doing searches here on FPN would return many threads discussing this; internet searches would fill out the picture either further. The topic has been covered many times just here on FPN, and the mods and members of the repair forum have offered many thoughts.
The issue is that the original material - hard rubber - does not respond to solvent welding and other tactics commonly used in repairing celluloid, acrylic and other pen materials. On top of that is both the commonality of the lip cracks (a thin part of the pen) along with the fact that it would be cosmetic at best, and you could never post the cap again (which is where the caps usually spring from).
I'd urge you to spend a little time doing searches on "repairing hard rubber pens" or something similar. I'm not saying that someone might not come up with something, newer bonding agents my pop up that work, but in general, the consensus has been that this is one of the areas of pen manufacture and history that is hard to restore. The good part is that the pen in question can be somewhat commonly found and a replacement cap won't be particularly hard to source (pen shows, vendors (I'd try Five Star Pens to start), eBay).
Hope that's long enough.
"When Men differ in Opinion, both Sides ought equally to have the Advantage of being heard by the Publick; and that when Truth and Error have fair Play, the former is always an overmatch for the latter."
~ Benjamin Franklin