I think it is with the 2e Blood War development.
From the 2e Pre-Planescape
Outer Planes Monstrous Compendium appendix:
Bold and cunning, mariliths are the generals and tacticians for the
Blood War. Where the balors are the influence and energy behind
the war effort, the mariliths are the brains and tactics.
Terrible to behold, these creatures appear as half snake, half
humanoid. From the waist down, these creatures have the bodies
of giant snakes, with huge, green, scaly coils. From the waist up,
they are beautiful, comely females. Besides the obvious snake
body, the feature that gives away the true nature of this tanar'ri is
her six arms. They are commonly adorned with precious jewelry
and highly decorated weapons—they most often carry a different
weapon in each of their six hands.
. . .
Habitat/Society: As noted, mariliths are the strategists of the
Blood War. They are in charge of devising all tactics to be used
during the battles and coordinate the activities of all true tanar'ri
(save the mighty balors, of course). Due to the chaotic nature of
the non-true tanar'ri, it is not really possible to coordinate their
activities, but the mariliths are charged with it nevertheless.
These stunningly powerful creatures have a hatred for the glabrezu.
They feel that the subtle warfare of the glabrezu is a childrens'
game and inferior to proper warfare. A marilith will
attempt to discredit the workings of the glabrezu any chance they
get. They would completely destroy the subtle beasts, but the allpowerful
balors favor the glabrezus' tactics and would have them
continue their ways.
Ecology: Mariliths are true tanar'ri, which makes them a part of
the driving force behind the Blood War. Moreover, they are the
coordinators and generals of the war efforts. Mariliths feel that
they are the true force behind the war. Secretly, they feel that the
balors have their place only because of their great power but are
not absolutely necessary. They may be right in their belief, but
the power of the balors remains unchallenged.
I just did a search of the 1e Manual of the Planes and did not get any hits for "type V" or the individual type V "Marilith". This was a pre blood war product.