It's definitely bronze or brass -- catch the light on some of the edges. "Really tarnished" would be almost black for bronze or brass.
A lot of "Stanley" planes and such are found in brass/bronze, though almost none were offered as such by Stanley. I've heard a number of stories about how they came to be, but nobody seems to be really certain, or able to document the source. One such story is that Stanley workers made the brass ones for fun in the after hours.
Jim C, do you have any ideas? Different, probably apocryphal stories?