  Antiferromagnetic compounds chromium sesquioxide (Cr$_2$O$_3$) and
dioxomolybdenum selenite present linear magnetoelectric effects. Anti-inversion
symmetry in the corresponding magnetic crystal classes dictate the makeup of
magnetic Bragg diffraction patterns. Copper axial and polar magnetic multipoles
contribute to resonant x-ray and magnetic neutron amplitudes in a symmetry
informed analysis of monoclinic Cu$_2$(MoO$_4$)(SeO$_3$) presented with a view
to steering future diffraction experiments. The compound might be viewed as a
low-dimensional quantum magnet on account of its crystal structure and extreme
Cu spin (S = 1/2).
