Fantastic research and results! I typically avoid yellowed hardware I see on eBay (it often goes for less as well). Maybe now I can save some money de-yellowing it myself.

Until now, I thought the the only solutions were to mechanically remove the top layer of plastic (i.e. SoftScrub abrasive cleaner) or painting over it with paint that's a "close enough" match. Good to know there's a better solution.