That part is a transmission axle from a gear in the right side at the bottom of the paper feed roller assembly, behind a oval like hole, that connects there with a gear in one extreme of this axle, the one that has the white piece. The axle sits in two U holes in the left and central walls, of the compartments that are under the printer head cleaner tray (see Step 34 in the guide for Feed Roller Assembly Removal, to remove this tray). The other end of the axle, that has a pinion, connects to a rack in the under side of this tray, in order to move the tray forward and back.