  These notes merely intend to memorialize Souriau's overlooked achievements
regarding geo\-metric quantization of Poincar\'e-elementary symplectic systems.
Restricting attention to his model of massless, spin-$\half$, particles, we
faithfully rephrase and expound here Sections (18.82)--(18.96) &
(19.122)--(19.134) of his book \cite{SSD} edited in 1969. Missing details about
the use of a preferred Poincar\'e-invariant polarizer are provided for
completeness.
