TWA still flies the DC-9-51 series; a couple of the STL-SAT flights are on that type equipment.

Splitting hairs, though, but the FAA considers the MD-80 series a DC-9 (Series 81 through 88), since it was originally certificated as a variant of the DC-9. (The MD-90 and the MD-11 were certificated under the MD-XX moniker.)