An exceedingly simple product. 
 
This tube aligns your Empire-spec (or in most cases, Eclipse-spec) clamping feedneck to the feedneck block on a modern Sterling STP. This is by far the easest way to add a clamping feedneck. 
 
Installation is simple - remove the through-screw from the STP feedneck block. Remove the stock feedneck. Set the new, clamping feedneck on top of the block, with its bottom screw loosened or removed. Drop this adapter in from the top. Reinstall the screws, and your feedneck is mounted! 
 
Photos show a J4 Torque stock feedneck mounted to a Sterling STP.