Tentative answers to my own questions. "S" apparently denotes "Smith," who was a pioneer in the development of interference contrast. "T" denotes "transmitted" light interference contrast. "R" denotes incident or "reflected" light interference contrast.
Andre, Mike. 2006. Leitz Orthoplan Universal Largefield Research Microscope. A brief illustrated history. 8 pp. www.microscopy-uk.org.uk/mag/artmay06/ma-orthoplan.html
Patzelt, Walter J. 1985. Polarized Light Microscopy. Principles, instruments, applications. 3rd edition. Ernst Leitz Wetzlar Gmbh. 103 pp. earth2geologists.net/Microscopes/documents/Leitz_PolarizedLightMicroscopy.pdf