Let
$F$
be an infinite field of positive characteristic
$p > 2$
and let
$G$
be a group. In this paper, we study the graded identities satisfied by an associative algebra equipped with an elementary
$G$
-grading. Let
$E$
be the infinite-dimensional Grassmann algebra. For every
$a$
,
$b\in \mathbb {N}$
, we provide a basis for the graded polynomial identities, up to graded monomial identities, for the verbally prime algebras
$M_{a,b}(E)$
, as well as their tensor products, with their elementary gradings. Moreover, we give an alternative proof of the fact that the tensor product
$M_{a,b}(E)\otimes M_{r,s}(E)$
and
$M_{ar+bs,as+br}(E)$
are
$F$
-algebras which are not PI equivalent. Actually, we prove that the
$T_{G}$
-ideal of the former algebra is contained in the
$T$
-ideal of the latter. Furthermore, the inclusion is proper. Recall that it is well known that these algebras satisfy the same multilinear identities and hence in characteristic 0 they are PI equivalent.