I've used their ammo at matches in the past. Worked fine and was cheaper than Federal by about half back then. The cases at the time were Winchester. But when you order a large enough number, you can get your own head stamp. I think Starline told me forty or fifty thousand case minimum for that, but it was a long time ago. Forty years ago, Winchester had a special deal where they would put your name on the head stamp for a 2,000 case minimum, but it was just a short-lived promotion. Nonetheless, again, if you order enough you can get what you want on the head stamp. So HSM could still be Winchester brass.
The discoloration from annealing is called annealing stain. The military requires it to be visible on cases to prove they have been annealed to prevent season cracking. Also, Hatcher did tests on corrosion resistance that showed leaving oxides on the case made them much more resistant to corrosive atmospheres than when they were polished. The reason for polishing is just appearance, though I find it helps when I have to find the ejected brass in the grass during a short policing call.
You can see a picture of brass with an annealing stain here:
https://www.midwayusa.com/product/2090862857?pid=898842
Let us know if that matches what you see.