It says in the text: "The powers of the cube that Nex kept for himself were a closely guarded secret, but rumor has it that with his cube, Nex could block incoming magic at will while still casting spells outward. While modern magical scholars believe this effect to be impossible, much of the magic of Nex is shrouded in the seemingly impossible."
That is under the "Cubes of Force on Golarion" in the book. That means that there is only one Cube that does this and no other, the cube that Nex had. The Cubes of Force that Nex made, which is more than one that has different kinds of magical power, not the same as the cube mentioned earlier, has fallen into the hands of the Council of three and Nine, and some remain abroad according to the text.
We may have not been following the rules of a Cube of Force in 3.5. The reason why I say this is that it mentions this power so matter of factly as if everyone should know this.
