Actually, if you zoom in all the way, the ring on the zipper of this particular NYC Basic Bag appears to be a common split ring replacement and not the original (either "paperclip" or solid hoop variety). Look at the seam running around the circumference.
With the exception of the zinc pulls that were used on the interior zippers of some 1970's bags and the alloy used on the "paperclip" style retainers that appear on some of the early clutch handles/pulls, I'm hard pressed to see why
COACH would have employed anything but brass, especially on exterior hardware, given corrosion concerns (kisslock frames perhaps being an exception). I have seen some rectangular handle retainers (the noncontinuous variety where the break was hidden in the center under the leather loop) on older bags and briefs that did not at first appear to be brass, but a quick test with a file showed that they were. As with anything COACH, though anything is perhaps possible. Doubly so for supplier sourced components.