I checked all my sources for NOS parts, and these covers have been discontinued for so long that there is nothing out there in cyberland! The original black part number is D1ZZ-6561694-A, replaced by D1ZZ-6561694-H. I checked all the other colors just in case any showed that could be painted, but nothing came up with those numbers either. The '72-3 Mustang used D2ZZ-6561694-A. The Mercury Cougar and '71/ Maverick with the bucket seat option used the same hinge cover.
If no one here has one looking for a new home, you could check with Mike @Motor City Mustang. He has been known to pull a magic rabbit out of his hat more than once.
Another source for a used cover would be West Coast Classic Cougar.
Used RIGHT side bucket seat black hinge cover, for any 1971-73 Mercury Cougar or Ford Mustang. This is for only a single hinge cover in used condition. Most will have minor scuffs or imperfections, common colors in shock include: ginger, green, and...
Thanks for the info. As per your suggestion, I contacted West Coast Classic Cougar and was able to order a Ford used one from them. Their price was reasonable (I thought), especially since the after market covers do not have the raised boss to fit / retain the cover to the side of your hinge.