Comparison of the +293d nebular spectrum of SN 2021aatd to the model spectra published by Jerkstrand et al. (2012, 2014). Model spectra with the closest epochs to that of the observed one (+306 days for the 12, 15, and 25 M_⊙ models, and +332 days for the 19 M_⊙ model) were selected; see details in text. All spectra are normalized to the flux values at 6000 Å. The [O I] line doublet (λλ6300, 6364) for the model and the observed spectra is shown in the inset; the observed line profile resembles most closely the 15 M_⊙ model, suggesting that the initial mass of the progenitor is close to this value.