  A special form of the $3 \times 3$ Majorana neutrino mass matrix derivable
from $\mu - \tau$ interchange symmetry accompanied by a generalized $CP$
transformation was obtained many years ago. It predicts $\theta_{23} = \pi/4$
as well as $\delta_{CP} = \pm \pi/2$, with $\theta_{13} \neq 0$. Whereas this
is consistent with present data, we explore a deviation of this result which
occurs naturally in a recent proposed model of radiative inverse seesaw
neutrino mass.
