Off-center button with locating pin
This button on a Ceruti has fairly normal proportions for a violin. The convention is for the button to be symmetrical about the center-joint of the back. In this case however, the maker failed to align the center joint with the back’s axis of symmetry.
Notice small cracks on either side of the button. As a wild guess I would say that at least 50% of older violins have this or worse.
The small pin below the purfling was used while constructing the violin to help relocate the back onto the ribs. Other pins seen in examples below are usually later additions by repairmen.