The AD&D DMG states that wearing a 'great helm' gives the head an armour class of 1, along with limiting the field of vision of the wearer to 60 degrees. But it does not specify (at least, I couldn't find specifications) the AC the head receives from a regular helm, or how restrictive said helm would be to vision (I assume it has no consequence, but am unsure). So how much AC does a helm give the head, and does it restrict vision?
Just prior to your citation (DMG 28): "It is assumed that an appropriate type of head armoring will be added to the suit of armor in order to allow uniform protection of the wearer."
Uniform Protection = Same AC as the armor worn; No helmet means head is AC10 (which is targeted 50% of the time -- 1-3 on a d6 -- when fighting an intelligent foe). There is no specification of field of vision in the text and I agree with your interpretation that there is no penalty.