This is a genuine Porter Cable replacement part, it is sold individually. A Cord Strain Reliever is used on most all power tools. It purpose is to reduce the bending of the cord as it comes out of the housing, which otherwise would cause the cord to crack and wear.
This item has been sourced from the original manufacturer and is intended to be used with belt sanders from Porter Cable. The metal drive pinion is used to link the motor gear and the large pulley to allow the two parts to rotate in unison. This item is sold as an individual replacement part.