  The periodic flag manifold (in the Sato Grassmannian context) description of
the modified Korteweg--de Vries hierarchy is used to analyse the translational
and scaling self--similar solutions of this hierarchy. These solutions are
characterized by the string equations appearing in the double scaling limit of
the symmetric unitary matrix model with boundary terms. The moduli space is a
double covering of the moduli space in the Sato Grassmannian for the
corresponding self--similar solutions of the Korteweg--de Vries hierarchy, i.e.
of stable 2D quantum gravity. The potential modified Korteweg--de Vries
hierarchy, which can be described in terms of a line bundle over the periodic
flag manifold, and its self--similar solutions corresponds to the symmetric
unitary matrix model. Now, the moduli space is in one--to--one correspondence
with a subset of codimension one of the moduli space in the Sato Grassmannian
corresponding to self--similar solutions of the Korteweg--de Vries hierarchy.
