Stradivari, yer basic button
The button normally falls on the center joint of the back, Often you will see pin prick at its center from the lay out process. The edge of the button is usually finished with a chamfer in this example by the Grand Master Himself, most of the chamfer has been eroded, probably during repair work, but it is still visible at the edges. The end of this chamfer has been emphasized with a notch. It gives the violin a certain stylistic integrity if the shape and chamfer style of the button reflects that of the chin of the scroll.
An interesting side detail here are the marks on the back of the neck heel which could be from the nails that Strad used to attach the neck to the ribs. That would indicate an original Strad neck which would be very unusual as most necks from that period have been replaced. The extensions visible on the sides of the heel would support the original neck theory.