Nice job!
My guess was an aluminum alloy but I might be wrong.
My machinist has been advocating machining the corners off and anodizing right from the start. I'm working on the next batch right now and I'll be anodizing some of them black for an upgrade price. I'll keep the corners though. Helps prevent it from rolling around in the trunk!
I'm curious Jory, did they make you take the pins out before anodizing? When I looked into this recently they told me I would have to anodize it before pressing the guide pins because they don't want anything but aluminum in the bath. They said ss would contaminate it. Having to set up after anodizing to machine it an extra time for the pins adds to the price of anodizing.