The volume of the forward tank is the same on a MK1 and a MK2. A MK2 has the forward bulkhead further aft to compensate the loss of volume. Please note there is a lot of space between the mast and the forward bulkhead on a MK1 and there is none on a MK2.
The foam, as you noticed, is there to prevent the boat from sinking should the tanks flood. (GRP is heavier then water). Actually the foam in the sides makes the boat harder to recover after a capsize but because the MK2 was designed as a pure cruiser this is a compromise to provide more storage space in the aft tank. Should the tanks be flooded the foam will prevent the boat from sinking but it cannot be bailed anymore because the top of the CB case stays submerged. And that is why safety demands us to check our tanks every year and keep them closed at sea, at all times.
And thus I avoided the words “buoyancy” and “locker”.