Double End Stud 1/4" Inch and 3/8" Inch Adapter Spigot for Flash Light Stand - Walmart.com
Spigot adapter 16 mm w/ 1/4" & 3/8" threads
Amazon.com : 2 Pieces Standard 1/4 to 3/8 inch Metal Male Convertor Threaded Screw Adapter Spigot Stud for Studio Light Stand, Hotshoe/Coldshoe Adapter, Ball Head, Wireless Flash Receiver, Trigger : Electronics
6pcs Spigot Stud Adapter Male Convertor Threaded Screw Standard 1/4 to 3/8 inch Metal for Hot Shoe/Cold Shoe/Studio Light Stand|Follow Focus| - AliExpress