This triangular spacer plate is fitted between the pop-out window lock and the bodywork.
Not all Volkswagen Buses were fitted with them at the factory, a reason we unfortunately couldn't find, but it seems to have been a modification made to Buses that had problems with the window frame sealing.
Fitting this spacer plate brings the lock closer to the window frame, allowing it to be tighter and provide a better seal.
It doesn't really matter whether your Bus is fitted with a spacer plate or not - fitting one will make your window frame much tighter against the bodywork than without.
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