Die Cast Zinc Insulated Conduit Nipple 4″, NPSM Threads
https://hotfix.nsiindustries.com/product/die-cast-zinc-insulated-conduit-nipple-4-npsm-threads/
The Bridgeport 4″ Die Cast Zinc Insulated Conduit Nipple serves as a short raceway between enclosures or boxes when used with a locknut. Conduit nipples are used to extend a conduit system. This nipple features NPSM threads.