These Agilent Varian VHS-400 diffusion pump jet coupling part, it is part of the jet assembly and is a OEM Agilent Varian replacement part for the VHS-400 diffusion vacuum pump. See referenced on Page 42 in instruction manual PDF below. These are original OEM Agilent Varian replacement parts for the VHS-400 diffusion pump having part number L6367301, see item #5 in the diagram exploded view & parts illustration below.