Starting in 1973, Volkswagen removed the wiper arm mounting nut from the field of vision of almost all VW models by hiding the nut in the wiper arm instead of mounting it on top.
A standard M6 nut and a corrugated spring washer provide the mounting and the plastic cap mentioned here for the final finish. In addition, the plug also guarantees that no moisture can penetrate.
The inside diameter of the plug is indicated as an extra in case you wish to use it for other purposes.
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