This is not uncommon. Almost any RWD from GM with rear disks made after the 1990s has a drum park brake. I have several examples, including B-body Impalas, S-10 trucks, and Envoy. The replacement rotors I've used all had the drum machined in the rotor hub.
The rotor OEM part number is likely 19241878 or 88875212.
The harder part is finding the one-piece park brake shoe since not many places seem to stock them. As an example, AutoZone 55057 is shown as a replacement rotor, and they list the shoe as #781.
The brake shoe part numbers are probably AC-DELCO 17771B, 14771B, or 88997309.