It appeared that all of these rock tombs at one time had tablets like this one, which give the names of the people who are buried there, who built the tomb and other information that I now can’t remember. The tablets are written in the Nabataean alphabet, a consonantal alphabet that descended from the Aramaic alphabet via the Syriac alphabet. A cursive form of it in turn developed into the Arabic alphabet from the 4th century. (from Wiki)

Mada’in Saleh is one of Saudi Arabia’s two UNESCO World Heritage Sites. The city was settled by the Nabataeans, whose capital was Petra in Jordan, in the first century B.C., and most of the 131 rock-cut tombs date to the first century A.D.