Sounds like a variation of the Hull Down rules from Western Desert, p161-162.
If the vehicle has a turret, only turret mounted weapons can fire. If turretless, you may choose one weapon to fire.
I haven’t bought into Achtung Panzer so can’t comment if it’s from there. Another possibility would be a rule we haven’t seen yet from V3 – where did you see this ‘turretless’ rule?
