We recently discussed the manufacture of hard rubber taper caps, and specifically how they were made by wrapping raw rubber around a steel mandrel, the rubber then being vulcanized in place. Since the unvulcanized rubber is dough-like, and not a liquid, there inevitably has to be a seam where the edges of the rolled-out rubber sheet meet. This seam is normally invisible after the cap has been vulcanized, turned or ground to final dimensions, and polished, though in some cases a telltale line of porosity can be seen -- usually on second-tier pens, where quality control apparently was not quite so strict. The seam can also become visible if the hard rubber ends up light-faded, as illustrated by the photo above (click to enlarge). In such cases, the rubber has proven more resistant to fading where it has been joined.
The first time I noticed this phenomenon was on the cap of a giant Moore safety pen in a Chicago pen show auction, long ago. At the time I could not decide if the line meandering down the side of the cap was some sort of manufacturing flaw or the traces of an extremely skilful repair. The answer only became clear years later, after seeing several other examples, reading more about how hard rubber caps were made, and finally putting all the evidence together. Once you start looking for that seam, you'll find that it's often visible but easily overlooked. This also applies to mottled hard rubber, where the seam may hide in plain sight in the material's irregular patterning.
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