  Ultrarelativistic isobar collisions have been proposed as a useful tool to
investigate nuclear structure. These systems are not created in equilibrium,
rather undergo a prethermalization phase. In this phase, some of the initial
structure information may be lost and additional effects introduced. The
objective of this paper is to study this possibility in the extreme case of a
"free-streaming" pre-equilibrium phase. We do this by computing estimators for
ratios of various measured (or measurable) quantities (elliptic and triangular
flows, mean transverse momentum and associated cumulants, correlators between
elliptic or triangular flows and mean transverse momentum, symmetric cumulant
and two-plane correlator) and study their sensitivity to the duration of the
free-streaming phase. We find that the correlators between elliptic or
triangular flows and mean transverse momentum, the so-called $\rho_2$ and
$\rho_3$, are indeed sensitive to the duration of the free-streaming phase and
that the normalized symmetric cumulant, $\epsilon NSC(2,3)$ might also depend
on this duration.
