The 'I' used for body-centered systems is for the German word Innenzentriert and the 'F' for face-centered is for Flächenzentriert, also German but what about the 'S' for a, b, or c base-centered crystals?
S also has German origins: seitenflächenzentriert (side face centered).
Reference: International Tables for Crystallography, Volume A, Space Group Symmetry, Fifth Edition, Springer 2005, pg. 748.
One can trace a 1922 article: Niggli, P. "XII. Die Kristallstruktur einiger Oxyde I." Zeitschrift für Kristallographie-Crystalline Materials 57, no. 1-6 (1922): 253-299
A German speaker can identify if this Fraktur S or C. Hard to assess for me.
In general, beware of symbolic stories. The above reference is authentic. Sometime, people tend to invent stories about symbols. My recent discovery was that of a sinc function. The whole web and plenty of books call it sinus cardinalis. It turned out to be a fictitious story. Similarly, most of Schoenflies symbols are not his inventions, and neither are Mulliken's symbols his.
1$\begingroup$ That is a C :) You can check it, e.g., using the Unifraktur Font on Google Fonts: fonts.google.com/specimen/… $\endgroup$ Mar 21 at 8:25