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Pawel, are you referring to that round feature at the bottom of the cap? That also seems to exist on some OEM caps, for example, the one pictured below. (I'm assuming it's original, just from its weathered appearance)As far as rumours of installing a double-sided key barrel into a cap intended for a single-sided key barrel, it would be great to hear from someone who's actually done this. Generally, double-sided barrels (for doors etc) are larger diameter than single-sided ones. So for example, there's no way you can install a double-sided barrel in a door handle originally intended for single-sided key because of the differing diameters. The two-sided barrel is bigger to accommodate wafers on both sides of the key.