The arbor is the post the cylinder rotates on. It goes into a blind hole just below the bore on the barrel assembly; it should bottom out in that hole just as the lower part of the barrel assembly contacts the main frame (where the two small locating pins go into the small holes on the barrel assembly). If the arbor is too short the barrel assembly can be bent upwards by the force of driving the wedge into place and the forcing cone will contact the cylinder face.
This is a very common problem. Most of the contemporary Ubertis, and a good number of Piettas, have it.
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