Magnetic diffraction Systematic Absences for the group P21/m' (#11.53) For this space group, BNS and OG settings coincide. Its label in the OG setting is given as: P21/m' (#11.4.62)
Values of h, k, l:  h integer, k integer, l integer
Systematic absences for special reflections:
Diffraction vector type: (0 k 0) -> Systematic absence: k = 2n