When I earned my M.A. at the University of Texas at Austin in 1995, the M.A. was the option for coursework and original research in the form of a thesis. The M.S. was reserved for the coursework-only option (rarely awarded).
In 1996 (I believe), the department changed the thesis-based Master's to an M.S. degree and did away with any no-thesis degrees higher than Bachelor's. This decision short-sells the awardee, unfortunately.
In general, an M.S. in STEM fields indicates a coursework-only degree (typically 6 classes, no thesis or original research) and an M.A. indicates a more rigorous course of study and research.
My Master's thesis, btw, is "Desorption of High Explosives from Soil: Thermodynamics and the Role of Soil Organic Carbon."