The port for the 16-50 has 67mm threads on it's end. if you're looking for the more convenient Nauticam 67mm flip holder. I think Nauticam makes the better flip because theirs has a spring lever lock that prevents heavier diopters from swinging away from the lens in downward positions. I looked for over a year for a used one, but they practically don't exist, as divers upgrading their rigs hold onto them for their upgraded package. I finally ordered one currently on clearance sale out of Singapore at Fun-In Photography. Nauticam sets the factory price at $260 (not cheap), but Fun-In currently has it on close out sale for $170 + S$12 shipping (to USA), basically getting a new one for used price. It takes a couple weeks though at the lowest shipping cost and I'm still awaiting delivery.