Dark Metallic Markings on Device

Hey everyone, I’m wondering if anyone else has run into this problem and knows how to resolve it. I’ve been wearing the Jailbird for three days non-stop with a black plastic-covered Master Lock attached. When my wife removed the device tonight, there was some dark metallic residue on my shaft right around where the two pieces join. I doubt very much that it’s just the plastic layer wearing off and leaving behind the mark. Any ideas?