My vacuum chamber which I am setting up for thermal evaporation has a large rectangular flange which has grooves for ISO style clamps and 2 locator pins. This flange may have been used for something like a load lock but as far as I could tell it is not a standard size so a custom block off plate was designed and machined from SS with grooves on the back for clamps, 2 pressed locator pins, and a viton o-ring. Mating surface was hand lapped to ensure good vacuum seal. Made by the extremely talented J.D Guerard machining company.