Quill Stem AdapterTransx brand. Adapts threaded 1" (25.4mm) quill to 1 1/8" (28.6) and allows you to change your quill stem over to ahead 28.6 mm stem.
The wedge end of the adaptor is 22.2mm diameter to fit into the 25.4 outer diameter steerer tybe (= 22.2 ID).