Was "Black" an RPO color for 71 only? Is that why you asked for special-order 72/73 cars?
If so, that would help to explain why they seem so rare.
Years ago, someone I knew casually who claimed to have worked in a Ford plant told me that Ford did not like to paint cars black unless customer-ordered. Supposedly, only those bodies with above-average fit & finish would be "picked" for black paint, as that color showed all the flaws.
That always seemed like BS to me. Isn't the car "bucked" with a VIN and equipment list ( including paint color) long before it is actually built and ready for paint?
If so, how then could a "random" body be picked for black paint?
I have owned four brand new black Fords: 82 Mustang GT, 85 Turbo Coupe, 87 Turbo Coupe, and a 90 Super Coupe Anniversay car.
All of them had typical fit & finish...meaning: Not very good. Runs, orange-peel, bad panel gaps, crap body welds, etc...
The 82 GT was the worst: Bubbles in the paint, runs and hazy spots, badly aligned panels. And crazy orange peel...never seen worse.
But it is just typical of mass-produced cars of thier day.