Game Version number: 1.3.0.170
Modded: No

Issue Description:
BTR70_FIA.et is missing said ammo storage prefab, which is added in the SlotManager of BTR70.et, instead of BTR70_Base.et.
I recommend moving the vehicle part to the BTR70_Base prefab, or alternatively also adding it explicitly to the BTR70_FIA SlotManager.
The prefab is not just something visual, it also contains a HitZone which would currently be missing on this BTR variant.