This model is based on the prototype produced in early 90s. With a technical gimmick making difference in level on the face, on the side and on the back of the front, this frame looks like a Sirmont frame. Furthermore, there is difference in level at backside of end-piece to fix a hinge with a rivet. This model is made with consolidated technologies of Japan. 307, the simple model in a same lens shape without difference in level, is also available.