this is a Sagadive port made for Canon ef 100 mm
this port is mounted directly on the housing without additional extension ring (replacing the RDX Port Base S + Compact Macro Port 111)
it is made of Delrin (very light and durable) the lens holder Sagadive fit perfectly and the lens is closer to the lens wet perfect condition, no scratches on the glass, some unavoidable traces at the lens holder price: 110 euros
(on the right of the second pictures)