Observed and modelled maximum brightness temperatures (circles), and integrated intensities (squares) for the blue-shifted emission. Data points (in black) are corrected for an emission region of 3 arcsec² and for 60% of the emission due to OF1. Errorbars are ±20% of the observed value. Three models are shown: the model of Paper II with level populations in statistical equilibrium (“s-e” in red) with ν_s = 32 km s^-1, one with a slower shock velocity (ν_s = 30 km s^-1, blue), and a model in stationary-state (“s-s” in green).