There are several ways to put NPT (National Pipe Thread, which is a tapered, sealing thread used in plumbing; it is the US standard for pipe threads) in a drum, tank, or pressure vessel. The obvious is, of course, to use or adapt any threads already provided on the vessel. Another is to drill and tap a pipe-threaded hole, although this only works if the wall of the vessel is sufficiently thick to create enough threads; often it is not. A third is to use what's known as a through-wall fitting, which assembles around a hole drilled in the tank and does not require welding. These are rated for mild pressure and can be useful. And finally, there is the item pictured, a weld-on tank bung.