Lost in translation: Pluma ibérico
As far as I can tell the "pluma" consists of the cervical and thoracic rhomboid muscles.  Those muscles are not standard cuts in the US.  It would likely remain attached to the whole loin or as part of blade end loin chops.

The article you linked to states that "the pluma should not be confused with the blade end of the loin or the 'butcher's filet' which is the caudal (tail-ward) cut of the loin."

There is a video on how the pluma is butchered available on youtube: http://www.youtube.com/watch?v=3H_HNjStwsk