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Ball-milled Li₃MCl₆ solid electrolytes conduct better than their annealed counterparts, an advantage attributed to "stacking faults" without the defect being identified. Synchrotron diffraction (λ = 0.2073 Å) and impedance spectroscopy, recorded on one Li₃TmCl₆ specimen through a thermal cycle, identify it. In the 3a × 3a cation superstructure the reflections divide exactly by h + 2k mod 3: those with h + 2k ≡ 0 stay sharp, the rest are three to five times broader, with only mixed peaks in between. That index is the phase a reflection acquires under the in-plane translation (1/3, 2/3), so the defect is a translational antiphase boundary of the cation order, leaving the chloride close packing undisturbed. Measuring it required modifying FAULTS: the DIFFaX recursion cannot represent a boundary inside a layer, so a class-selective pair of in-plane diameters was added where the program distributes each (h,k) rod into the pattern, refined with the structure in one least squares. At 157 °C it gives a fault probability of 1.20(8) % and ordered blocks 90 Å across and 510 Å tall. Across four temperatures the coarsening is strongly anisotropic. Blocks that begin equiaxed at 70 Å become columns of 93 × 204 × 646 Å, the walls perpendicular to c disappearing fastest, while the fault probability falls eightfold without reaching zero. The antiphase wall area per volume falls 2.4-fold over the interval in which the conductivity advantage collapses.