  Fossil groups (FG) of galaxies still present a puzzle to theories of
structure formation. Despite the low number of bright galaxies, they have
relatively high velocity dispersions and ICM temperatures often corresponding
to cluster-like potential wells. Their measured concentrations are typically
high, indicating early formation epochs as expected from the originally
proposed scenario for their origin as being older undisturbed systems. This is,
however, in contradiction with the typical lack of expected well developed cool
cores. Here, we apply a cluster dynamical indicator recently discovered in the
intracluster light fraction (ICLf) to a classic FG, RX J1000742.53+380046.6, to
assess its dynamical state. We also refine that indicator to use as an
independent age estimator. We find negative radial temperature and metal
abundance gradients, the abundance achieving supersolar values at the hot core.
The X-ray flux concentration is consistent with that of cool core systems. The
ICLf analysis provides an independent probe of the system's dynamical state and
shows that the system is very relaxed, more than all clusters, where the same
analysis has been performed. The specific ICLf is more $\sim$5 times higher
than any of the clusters previously analyzed, which is consistent with an older
non-interactive galaxy system that had its last merging event within the last
$\sim$5Gyr. The specific ICLf is predicted to be an important new tool to
identify fossil systems and to constrain the relative age of clusters.
