REDUCER 5/8" OD X 3/8" HOLE. 
This is considered an "odd" size. 
Lamp terminology can be tricky. 
This reducer has 3/8 IPS (refers to inside pipe diameter) thread on one end (5/8" outside diameter) and 3/8" hole 
(works with 1/8 IPS pipe). 
$2 minimum charge orders less than $20 
$.50 packing charge for each parts box.