Plug 1/16'', long hex, PEEK

Use a plug to close off an unused port in a valve, manifold, or column. Ordinary plugs consist of a zero volume nut with a ferrule made up on a solid rod. These special high pressure versions have the ferrule and rod machined as a single, solid piece.
A cap is essentially a piece of hex stock with a zero volume fitting detail machined into it, but with no through hole.