  Let $X$ be a K3 surface, and $H$ its primitive polarization of the degree
$H^2=8$. The moduli space of sheaves over $X$ with the isotropic Mukai vector
$(2,H,2)$ is again a K3 surface, $Y$. In math.AG/0206158 we gave necessary and
sufficient conditions in terms of Picard lattice of $X$ when $Y$ is isomorphic
to $X$. The proof of sufficient condition in math.AG/0206158, when $Y$ is
isomorphic to $X$, used Global Torelli Theorem for K3 surfaces, and it was not
effective.
  Here we give an effective variant of these results: its sufficient part gives
an explicit isomorphism between $Y$ and $X$.
  We hope that our similar results in math.AG/0304415, math.AG/0307355,
math.AG/0309348 for arbitrary primitive isotropic Mukai vector on a K3 surface
also can be made effective.
