Very Basic Question: Idler Pulley Differences

[cebra]

I bought an idler pulley from rock auto but it is quite different than the ACDelco one and the outside "cup" does not work with it but the small inside one does. Do I just tighten the bolt right against the bearing on this and use the inside cup only? It just seems odd to me the stock one comes with a "cup" on the outside and this one is very different. This may be a really dumb question but I don't want to break a belt on the road. Thanks. Pics below of ACDelco vs what I ordered...both are idler pulleys per rock auto.