These Agilent Varian VHS-6 & VHS-250 diffusion pump heater clamp assemblies are OEM Agilent Varian replacement parts with part number 86643301. These clampling plates help transfer heat from the diffusion pump heater to the oil inside the pump. these clamping plate kits include: clamping plate, cover plate, and insulator, see #11 abc below in the illustration below.