I am looking for the origin of the symbol $S_n$ used by chemists to denote the symmetry operation consisting of a $\smash{\frac{2\pi}n}$ rotation ($C_n$) about an axis and a reflection in a plane perpendicular to that axis.
Google Scholar traces earliest use of the operation’s name “Drehspiegelung” to A. Schoenflies’, Krystallsysteme und Krystallstructur (1891). However, that book does not seem to denote it $S_n$.
Physics books on molecular structure had $S_n$ by the 1930s: an early use can be seen in L. Tisza, Zur Deutung der Spektren mehratomiger Moleküle (1933, p. 59), so it must have started earlier.
(N.B.: Symbols like $\mathrm C_n$, $\mathrm S_n$ can also denote symmetry groups. This should not be confused with the notation for symmetry operations or “elements” — see e.g. this table.)