I would hesitate to describe myself as "more knowledgeable", just deeper into the complexities and confusion than most people.
There is a question mark in my mind over the "M" being for "military" for the following reasons.
1) The armoured cars are universally referred to as "1Z" for the early ones and "1ZM" for the later ones. An armoured car can never have been anything but military.
2) The "1Z" autocarri is sometimes described as a civilian truck with a militarised version designated "1ZM" but other sources say this truck was specifically developed for the military in 1912 during the Italo-Turkish War so , again, would only ever have been military. The other possibility, which would make sense in the context of the armoured cars, is that it stands for "modificato" as in "modified".
The various plans do indicate though that the "1Z" designation was still in use in 1918 well after the original truck is said to have gone out of production replaced by the Jota.