REDUCER 3/8" OD X 1/4-27 HOLE. 
Unfinished brass reducers have 3/8" outside diameter threads (standard pipe size 1/8 IPS) to 1/4-27 IPS hole (standard hole for harps ). 
Just screw into hole and they're ready to use. 
$2 minimum charge for order less than $20.
$.50 packing charge for each parts box.