  We revisit spin-orbit coupling in a plasmonic Berry metasurface comprised of
rotated nanoapertures, which is known to imprint a robust far-field
polarization response. We present a scattering formalism that shows how that
spin-momentum locking emerges from the geometry of the unit cell without
requiring global rotation symmetries. We find and confirm with Mueller
polarimetry measurements that spin-momentum locking is an approximate symmetry.
The symmetry breakdown is ascribed to the elliptical projection of circularly
polarized light into the planar surface. This breakdown is maximal when surface
waves are excited, and a new set of spin-momentum locking rules is presented
for this case.
