  In this study, we analyze the post-maximum spectra of a sample of 27 Type I
superluminous supernovae (SLSNe-I) in order to search for physical differences
between the so-called Type W and Type 15bn sub-types. This paper is a
continuation of \citet{ktr21} and \citet{ktr22}. In the former, it was revealed
that not all SLSNe-I show the W-shaped absorption feature between 4000 and 5000
\AA\ in the pre-maximum spectra, and two new SLSN-subgroups were disclosed:
Type W, where the W-shaped feature is present, and Type 15bn, where it is
missing. In the latter, it was shown that the pre-maximum photosphere of Type W
SLSNe-I tend to be hotter compared to Type 15bn objects, and they are different
regarding their ion composition, their early light curves and their geometry as
well. For completeness, post-maximum data are analyzed in this paper. It is
concluded that in terms of photospheric temperature and velocity, Type W and
Type 15bn SLSNe decrease to a similar value by the post-maximum phases, and
their pseudo-nebular spectra are nearly uniform. Pseudo-equivalent width
calculations show that the pEW of the wavelength range between 4166 and 5266
\AA\ evolve differently in case of the two sub-types, while the other parts of
the spectra seem to evolve similarly. It was found that the host galaxies of
the studied objects do not differ significantly in their star formation rate,
morphology, stellar mass and absolute brightness. The main difference behind
the bimodality of Type W and Type 15bn SLSNe-I therefore is in their
pre-maximum evolution.
