This plastic plug was originally used by Volkswagen to seal the 2 lugs above the rear bumper on both sides of the engine lid. Originally these lugs were used as access to apply the undercoating during production of the bus. These lugs were sealed by means of a plug later on the production process. 

When you remove these plugs they may be damaged or get lost during restoration of the bus. 

These caps are an exact copy of an original and thus have the correct diameter of 11.5mm.