On my submission feedback.
dostovel remarked that I had a six-sided pole on the back of the model. So I went searching for a solution to this. Below some screengrabs
When modeling or retopologizing a symmetrical object I make a habit of establishing a center loop that runs the entire circumference of the mirrored axis. Vertices at the exact center of the loop always meet in a four-pole and I don't change this as I'm working. For most of the things I do I don't need to touch things up at the end. However, depending on the mesh the center edge loop might not be required and could be dissolved after the mirror modifier is applied.