3-months old though? I wonder .... maybe worth contacting Pentair and showing them those pics just in case? They might try to point fingers at chemistry, but as long as your water isn't acidic and reacting to something metallic (which I doubt), I think they should have an explanation. I'd be curious to hear what they have to say.
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."