These are brand new, laser cut, dead soft 1000 series aluminum sealing washers specifically made for UG526 and RG3744 sump plugs - or any other 1 inch drain plug.  These fit all Early Post War, Silver Cloud, Shadow and Spirit oil sumps.  They also fit Shadow and Spirit final drive plugs.  They're cheap.  Get a bunch.  The vandal at your local Luby Stiff doesn't have these in stock and he's going to re-use your old one.