Sold as a complete unit. The sprung one has a body that sits slightly lower and further back on the engine than the pulley, it's fixed by a single hex bolt, as Tom says. The picture in this link shows it nicely: http://www.eurocarparts.com/ecp/c/Peugeo...181&000112
If you want a pic of it on the engine look in the aircon removal thread that's running in this section at the moment.
If you want a pic of it on the engine look in the aircon removal thread that's running in this section at the moment.