  The cumulant ratios up to fourth order of the $Z$ distributions of the
largest fragment in spectator fragmentation following $^{107,124}$Sn+Sn and
$^{124}$La+Sn collisions at 600 MeV/nucleon have been investigated. They are
found to exhibit the signatures of a second-order phase transition established
with cubic bond percolation and previously observed in the ALADIN experimental
data for fragmentation of $^{197}$Au projectiles at similar energies. The
deduced pseudocritical points are found to be only weakly dependent on the
$A/Z$ ratio of the fragmenting spectator source. The same holds for the
corresponding chemical freeze-out temperatures of close to 6 MeV. The
experimental cumulant distributions are quantitatively reproduced with the
Statistical Multifragmentation Model and parameters used to describe the
experimental fragment multiplicities, isotope distributions and their
correlations with impact-parameter related observables in these reactions. The
characteristic coincidence of the zero transition of the skewness with the
minimum of the kurtosis excess appears to be a generic property of statistical
models and is found to coincide with the maximum of the heat capacity in the
canonical thermodynamic fragmentation model.
