Corner plots showing the posterior probability distribution of key parameters for epoch 4 with a single P Cygni line fit. The fit parameters are (1) the cross-sectional velocity, v_⊥, derived from the blackbody normalisation fit assuming Planck cosmology (Planck Collaboration VI 2020), and several parameters of the 760 nm P Cygni feature, including (2) the photospheric velocity, v_phot, (3) the maximum outer ejecta velocity, v_max, (4) the velocity range, Δv = v_max − v_phot, (5) the optical depth of the line, τ, and (6) the central wavelength, λ₀, in μm. Velocities are indicated in units of the speed of light, c. These constraints show only the statistical uncertainty in the fitting model.