  The physical process whereby a carbon--oxygen white dwarf explodes as a Type
Ia supernova (SN Ia) remains highly uncertain. The degree of neutronization in
SN Ia ejecta holds clues to this process because it depends on the mass and the
metallicity of the stellar progenitor, and on the thermodynamic history prior
to the explosion. We report on a new method to determine ejecta neutronization
using Ca and S lines in the X-ray spectra of Type Ia supernova remnants (SNRs).
Applying this method to \textit{Suzaku} data of Tycho, Kepler, 3C 397 and
G337.2$-$0.7 in the Milky Way, and N103B in the Large Magellanic Cloud, we find
that the neutronization of the ejecta in N103B is comparable to that of Tycho
and Kepler, which suggests that progenitor metallicity is not the only source
of neutronization in SNe Ia. We then use a grid of SN Ia explosion models to
infer the metallicities of the stellar progenitors of our SNRs. The implied
metallicities of 3C 397, G337.2$-$0.7, and N103B are major outliers compared to
the local stellar metallicity distribution functions, indicating that
progenitor metallicity can be ruled out as the origin of neutronization for
these SNRs. Although the relationship between ejecta neutronization and
equivalent progenitor metallicity is subject to uncertainties stemming from the
$^{12}$C$\,$+$^{16}$O reaction rate, which affects the Ca/S mass ratio, our
main results are not sensitive to these details.
