Bridgeport NIP-200300 Rigid Conduit Steel Threaded Nipple 2″ X 3″
https://nsiindustries.com/product/bridgeport-nip-200300-rigid-conduit-steel-threaded-nipple-2-x-3/
The Bridgeport 2″ Steel Conduit Nipple serves as a short raceway between two enclosures or boxes.