Just had another look at the neck X-ray and enlarged it. If you examine the part I've circled, you can just make out an outline of something that appears to be solid and slightly darker than the lighter jawbone to the right of it. Maybe this is the part-whole remnant PV/20, and on this x-ray just didn't show up white and opaque like the metallic necklace and charm did. The whitish, round-edged fragments outside the circle resemble, and could be shattered pieces of vertebral bone (rather than bullet lead), which were propelled towards the front of her neck when the bullet hit, so appear closer to the camera and hence more prominent than the skull and vertebrae further away. In other words, the necklace, charm, and white bone fragments are in a similar plain, nearer to the viewer than PV/20, which lies further away close to the shattered 4th vertebra.
At the moment, I can't locate the colour photo of PV/20 to compare with the part circled.