Hmmm, I hope the inside of mine doesn't look like that. JamesW is on to something. I thought at first it might be the rivets or riveting process. But it's also streaking elsewhere. Second guess was the manufacturer's use of a lower grade metal (stainless, I'd like to think, but there are a lot of grades of stainless). Some attempt to shave a little more profit, no doubt. Maybe a change in supplier. Whatever it is, Pentain would be hard-pressed to claim it is anything but "a defect in material or workmanship."