Dereddened NLR ratios of [O III]/Hβ versus R_OIII for the same observational dataset as in Fig. 4. Two ionization parameter sequences of isochoric photoionization calculations with a frontal density of cm⁻³ are overlaid which assume either the Fg97 (magenta) or Ri14 (yellow) SED. A square identifies the U_o = 0.1 model. The third ionization parameter sequence (black dotted line) assumed the double-peaked La SED described in Sect. 7.1. Overlaid are two density sequences of isochoric photoionization models along which the density increases in steps of 0.5 dex, both having the same U_o = 0.3 value but assuming either the La SED (cyan solid line) or the accretion disk SED (light-green dashed line) from the OPTXAGN routine.