Yep, she's marring. This upper has over 1,500 rounds through it, but of that this tube only saw around 300 and was a virgin tube before the install.For comparison, here are the two POF tubes I use on my other T91s: this tube has roughly 1,000 rounds through it, all with one T91 upper this tube has roughly 1,800 rounds through it, the first roughly 1,200 were on one T91 upper, the remainder on another T91 upper
It should be noted that the second tube was originally on the aforementioned first T91 upper at the start of this post, then migrated to another lower to be replaced by the Aero. Also note the "wavy" pattern in the anodizing on the second tube, this was how it came from POF.
It's entirely possible that the marring on both the Aero and second POF tube is all from the first upper, but the round count to wear ratio on the Aero tube is shocking. Also entirely possible I just got a lemon tube.
In any case, buyer beware.