Evolution of the [O II] λλ 7320,7330 + [Ca II] λλ 7291,7324 line complex. Left: Up to t_max+565.0 days, the line complex is dominated by [O II]. The red wing narrows due to an increasing occultation of the CSM shell by the optically thick expanding SN photosphere. Right: Between t_max+565.0 days and t_max+637.2 days, the line complex shifts to the blue, consistent with the [Ca II] line becoming more dominant. All profiles are centred on [O II] λ 7320.