Welcome to the forum from North Carolina. I also have a Black 1973 mustang convertible. If you know our cars you will know that Black was not offered as a color option in 73. It could be special ordered as my car was. Special order cars will have a six digit DSO code and the front bumper and the filler pieces in front of the rear bumper that are rubber will be Argent in color. It will show up on the build sheet. A Marti cannot tell you much info on the Special order only that it was one of them. There were 277 special order verts in 73 for various options that Marti has no records of.
So if you have a six digit DSO I would like to talk in detail.