These are the mid-rings that will be used to bolt the two tube sections together. One is still in the mold - the rough surface will next be sanded flat with a belt sander, and then the circle jig will be used to put a rebate in the surface to complement the one you can see in the finished ring. This provides a keying mechanism to aid in positioning the sections of tube when bolting together, and also acts as a light baffle as well.