Corner plot indicating the posterior probability distributions of key parameters determined by fitting the spectra in epoch 1.1+1.2 (blue) and epoch 1.3+1.4 (red). The fit parameters are (1) the best-fit observed blackbody temperature, T_obs, (2) the cross-sectional velocity, v_⊥, derived from the blackbody normalisation, (3) the photospheric velocity of the 1 μm P Cygni feature, v_∥, and (4) the outer ejecta velocity inferred from the 1 μm P Cygni feature, v_max. The dashed grey line in the v_⊥ vs. v_∥ plot indicates the line of equality, i.e., a spherically symmetric velocity surface. Computing the cross-sectional velocity requires an estimate of the distance to the host; here we assume D_L = 44.2 ± 2.3 Mpc, as derived from the Planck cosmology (Planck Collaboration VI 2020), and the cosmological recession velocity of the host galaxy, z_cosmic = 0.00986 ± 0.00049 (Mukherjee et al. 2021; Sneppen et al. 2023b). We did not propagate the 5% uncertainty in the absolute luminosity distance into v_⊥, as we are interested in the relative change and uncertainties from fitting the spectra themselves. In the right panels, we illustrate the evolution in the continuum (top) and in the 1 μm absorption feature (bottom), which produces the subtle difference in the best-fit parameters. Additional systematic effects such as line-blending could shift the posteriors but are likely to affect sub-epochs in a similar fashion.