Dia-Compe 990's have a couple of versions of arms, particularly the portion at the top where the brake cable is attached. This barrel adjuster is for the type with a bolt on barrel adjuster. See 3rd photo for where this part mounts and the type of 990 arm it attaches to.
You receive exactly what is shown - barrel adjuster, barrel adjuster mount, nylock nut, and washer. When mounted, the barrel adjuster can pivot as needed when the brake is applied.