Just an idea, but I almost wonder if the extra pin was added after the fact, perhaps to halt the spread of a crack in the bone scale. It's a technique sometimes used to stop cracks from spreading in sheet metal, to drill out a hole at the crack location (or at the propagating end, if it's a lengthy crack), and fill it with a rivet or screw/bolt. Bone-handled knives are often-known to have cracks near the pins, and drilling & filling it might've been a 'stop gap' attempt to keep it from getting worse. Especially if the extra pin doesn't penetrate the inside of the liner; I see no function for it otherwise. I'd be shocked if that was actually one that was missed at QA in the factory. The pin even looks a different color, as if a slightly different material was used.