They are helium and hydrazine bottles. They have protective covers on them, that will be removed after the mate is completed.
The one with 203 on it is actually an Atlas III Centaur. One bottle for hydrazine and two for helium.
The long tank on the Atlas V Centaur was just a larger helium. It is for consolidating smaller bottles. There still is/are bottle(s) on the other side.
Thanks Jim, that was what I wanted to know!