  In this article we deduce two new exact solutions of Einstein's equations for
eternal black holes, now related to stiff matter, one `static' and another
rotating (stationary like the Kerr one), thus the number of these eternal
solutions grows from four to six. Dealing with these static and rotating
solutions we automatically rehabilitate the practically forgotten
Sommerfeld-Lenz approach to the deduction of our results for the satisfaction
of Einstein's equations, e.g., in these rotating cases. At the same time, we
come to the confirmation of our new relativistic generalization of the
inertial/gravitational mass equivalence principle, as well as to obtention of
the Kerr interior solution.
