Recently we found that canonical gauge-natural superpotentials are obtained as global sections of the {em reduced} $(n-2)$-degree and $(2s-1)$-order quotient sheaf on the fibered manifold $bY_{zet} times_{bX} mathfrak{K}$, where $mathfrak{K}$ is an appropriate subbundle of the vector bundle of (prolongations of) infinitesimal right-invariant automorphisms $bar{Xi}$. In this paper, we provide an alternative proof of the fact that the naturality property $cL_{j_{s}bar{Xi}_{H}}omega (lambda, mathfrak{K})=0$ holds true for the {em new} Lagrangian $omega (lambda, mathfrak{K})$ obtained contracting the Euler--Lagrange form of the original Lagrangian with $bar{Xi}_{V}in mathfrak{K}$. We use as fundamental tools an invariant decomposition formula of vertical morphisms due to Kolav{r} and the theory of iterated Lie derivatives of sections of fibered bundles. As a consequence, we recover the existence of a canonical generalized energy--momentum conserved tensor density associated with $omega (lambda, mathfrak{K})$.
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