Hm, perhaps I have a UN-72 because both drive and non-drive on mine are black annodized aluminum. I know when I worked at a bike shop we used to swap parts with other inventory to make an item whole. If it did not come in a box and was just the bare BB, they may have taken the non-drive cup for another BB. I have never seen a un-72/3 with a plastic cup but it has been a while since I worked at the shop too.