Time series of the velocity integrated 6.7 GHz maser flux density in IRAS 20216+4104 with the sinusoidal fitted dashed red line (top). The open circles denote the data from the HartRAO (in violet) and Torun (in green) radio telescopes. The squares mark the data from the literature (Table A.1) as indicated in the legend. Residuals from subtracting the sinusoidal fitting from the observational data are shown in the bottom panel. The black crosses mark the NIR continuum emission of the south-eastern outflow cavity (Massi et al. 2023, polygons S1 and S2 in their Fig. 2).