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." 

Lastly, older model Sharps Bros lower receivers had the rear spring/detent hole cut for a 4-40 set screw.  The engraving is very small but look on the back of your receiver before ordering.  It will be marked either 4-40 or 6-32 with an arrow pointing to the hole.  If you are looking for 4-40 set screws you can find them on amazon for cheap.  Just search "4-40 x 1/8 Inch socket head set screw."  

-JSharps