  We generalize a map by S. Mason regarding two combinatorial models for key
polynomials, in a way that accounts for the major index.
  We also define similar variants of this map, that regards alternative models
for the modified Macdonald polynomials at $t=0$, thus partially answer a
question by J. Haglund.
  These maps imply certain uniqueness property regarding inversion-- and
coinversion-free fillings, which allows us to generalize the notion of charge
to a non-symmetric setting, thus answering a question by A. Lascoux. The
analogous question in the symmetric setting proves a conjecture by K. Nelson.
