Die Cast Zinc Insulated Conduit Nipple 3-1/2″, NPSM Threads
https://testing.nsiindustries.com/product/die-cast-zinc-insulated-conduit-nipple-3-1-2-npsm-threads/
The Bridgeport 3-1/2″ 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.