Comparison of the oxygen abundance values derived via pyqz v0.8.1 (i.e., using the mappings v photo-ionization models; this work) and the O3N2 calibration of Marino et al. (2013), as employed by Sánchez-Menguiano et al. (2016b) for all the aperture spectra in HCG 91c. pyqz estimates span a wider range of oxygen abundances, meaning that pyqz-derived gradient slopes are steeper than those derived by Sánchez-Menguiano et al. (2016b). Despite a scatter of ~0.1 dex at the lower-abundance end, the correspondence between the two techniques remains nonetheless linear over the span of oxygen abundances in HCG 91c.