It is a quill mount. The other half of wedge is still binding bars to inside of steering neck tube. Loosen the large top nut above the white triple clamp in your pic. Unthread and slide upwards to reveal gap to spray more penetrating oil all the way around. Usually what I do when taking these apart is loosen the center bolt you removed a few turns then promptly smack the head with a hammer to unwedge before removing the entire assembly with bars as a unit.