[O I] λλ 6300,6364 and Mg I] λ 4571 lines at t_max+286.7 days. The Mg I] line is well fitted with a parabolic shape (dark blue), expected from an optically thick expanding shell (e.g. swept up CSM and unshocked SN ejecta), while the [O I] lines show a strong blue deficit because the line-forming region is still optically thick. The [O I] doublet is centred on the 6364 Å doublet component.