I believe they were both the v1 Heavy weapon guide and later in the basic rules at revision to give examples. It was one of the things the original rule set had pointed out, and adding them basic rules in later addition made sense to reinforce the point.
Honestly, I think any Militia or military units would keep their larger weapon in defensive position, even the 120 mm Mortars too. I can see these crews having responsibility of Medium or Light Mortars too that they take to the field for short duration operation in support of their units, depending on the needs. Like the Striker Brigade Mortar teams carry a lighter mortar that the gun crew can set up outside of the vehicle as needed. Even if the unit didn't have horses or other transport I can see where some troops could be pressed into service to move them by hand over limited distances.
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