Nebular spectra of SN 2020amv. Left: full range of the optical spectra, compared with SN 1993J (top, at 976 days, from Matheson et al. 2000) which also showed a boxy line profile, and with a sequence of spectra of SN 1998S (305 to 418 days, from Pozzo et al. 2004). Right: zoom-in on the Hα profile in velocity space. The profile is overall flat-topped and box-shaped in the first epoch, indicating emission from a thin dense shell formed when ejecta interact with the CSM. The later spectra show the development of an increasing overall asymmetry and structure, with a clear blueshifted horn at −5000 km s⁻¹. The evolution, with the suppression of the red side, is reminiscent of the evolution seen in SN 1998S.