[red-beard]

The M15 from Armalite used to be a single casting. They now use a universal upper with a rail and add a mounted carry handle.

From Brownells

http://www.brownells.com/rifle-parts/receiver-parts/receivers/upper-receivers/stripped-a2-upper-receiver-sku231000152-17601-41420.aspx?cm_mmc=cse-_-Itwine-_-pricegrabber-_-Ar-15%20Stripped%20Upper%20Receiver&gdftrk=gdfV21820_a_7c187_a_7c828_a_7c231000152_d_2 31000152_d_11224

Cheaper than Dirt

DPMS AR-15 Complete Upper Receiver .223/5.56 NATO 16" 1:7" Twist M4 Profile Barrel Flat Top W/Carry Handle Carbine Length Standard Handguards A2 Front Sight W/BCG and Charging handle Black BAAP416CH