It looks cnc'd so I think you're probably right that it's aluminum but I'm hoping it's a different grade or something. From brand new in the tube the body looked darker and as I used it the head and inner tube stayed bright and shiny even though I had my hands all over them as well. The body got almost black on the back where my hands were. I used a silver polish cloth on it but I'll take a picture in a few days or so when it gets noticeable...
I've been to the factory. They buy stock in ten foot 'sticks' to feed to one of three state of the art CNC machines (Swiss?). They only stock a few sizes, I'm sure the battery tube, sleeve, adapter, thermal core for T1 (body, core and cap) come from the same stock. Given the threads and thin profiles I'm pretty sure it's 6061 but the exact alloy doesn't matter since it's the same. And even if it's not, the Nickle plate is all you see.
BTW, they have 3 of those machines (some can only do the small parts like mouthpieces, switch parts and so on). It may take like 26 pieces to make a unit, but those machines turn out a part a minute give or take. There are buckets of parts, sometimes well over a thousand, all around. Being filled up, parts being cleaned, inspected, sent out for plating and so on. Structured confusion, it only looks like chaos.
They make a lot of vapes in that little shop. And an even bigger pile of e-cig stuff (I think).
Given the localized nature, I'm wondering if the oxide is caused by skin oils? Or at least helped along.
Good luck.
OF