We study the Wilson coefficients (WCs) for the radiative $B$-meson decays
$B \to X_s \gamma$ at the b-quark mass scale $C_7(\mu_b)$ and $C'_7(\mu_b)$
in the Minimal Supersymmetric Standard Model (MSSM) with general quark-flavour
violation (QFV) due to squark-generation mixing. We calculate the
supersymmetric (SUSY)-loop contributions to $C_7(\mu_b)$ and $C'_7(\mu_b)$ at
leading order (LO) in the MSSM. {\it For the first time} we perform a systematic
MSSM parameter scan for the WCs $C_7(\mu_b)$ and $C'_7(\mu_b)$ respecting all
the relevant theoretical and experimental constraints, such as the constraints
from $B$-meson data and the 125 GeV Higgs boson data, and the SUSY-particle mass
limits from the recent LHC experiments. From the parameter scan we find the
following: (1) The MSSM-loop contribution to Re($C_7(\mu_b)$) can be as large
as $\sim \pm 0.05$, which could yield a New Physics (NP) signal at about
3$\sigma$ level in the future Belle II and LHCb Upgrade experiments.
(2) The MSSM-loop contribution to Re($C'_7(\mu_b)$) can be as large as $\sim -0.08$,
which could make a NP signal at about 4$\sigma$ level in the future experiments.
In case such large NP contributions to the WCs are really observed in the future
Belle II and the LHCb Upgrade experiments, it could be the imprint of the QFV SUSY
(the MSSM with squark-generation mixing) and would encourage to compute the WCs
$C_7(\mu_b)$ and $C'_7(\mu_b)$ at NLO/NNLO level in this model.