All Sharps Bros. Gen2 AR15 lower receivers are threaded for a rear 6-32 set screw.  This set screw serves to capture the rear takedown pin spring which can make assembly and changing stocks down the road a little easier.     

If you would like to use one for your build, this is the proper one.  If you do choose to use a rear set screw: ** be sure to clip off a length of the rear spring equal to the length of the set screw, otherwise your rear spring will get over compressed inside the receiver and up against the rear take down pin. ** 

Please note: if they are out of stock on our website, or you want to buy them bulk, you can also find them on Amazon.  Just search "6-32 x 1/8-Inch Stainless Socket Head Set Screw." 

Priced per unit (qty 1).

-JSharps