"unity symbol" from Feller's "An Introduction to Probability" I am wondering if anyone knows the name or source of the symbol shown below from page 22 of the Second Edition of Feller's "An Introduction to Probability" Volume I. !enter image description here !enter image description here

Seems to be $\mathfrak{S}$, the Fraktur S.

Detexify is a great tool for things like this!