Is the blank spot in the disk, as in the bobbin has been lost, or a gap in your knowledge? If the latter, I would leave it. It's the mechanics of the unit that prevent the disk from warping with heat. You don't have to feel play in the connection by hand for it to work. In use, the rotor will expand with heat and get looser. When it cools, it gets tighter. The alignment difference you see is nothing to be alarmed about. There is no minimum specification on that. If the bobbins are undamaged, and the one pictured is undamaged, forget about it.