I've got the answer for you Stu. You should just send that damned old, stained Mobius piece to me to get it out of your sight. In all seriousness, looking at your pics, it almost looks like a water line. Especially the first one. And you can see splashes against the walls. I think this is old deposit. It's very possible that PBW isn't enough for older stains, but I would keep trying. On a straight tube, like HES', I would try a large aquarium brush. The bristles are soft enough that they wont scratch the glass. I used one on my straight tube to get at a resin spot under an ice pinch that wouldn't budge, no matter what I did. I just swished it at the end of the dwell time while the PBW solution was still in the tube, then gave it a super hot and long rinse. It worked beautifully.