49-55mm step-up ring. Allows the use of 55mm filter or attachments on lens with 49mm filter thread size.
Some digital and vintage lenses (eg Pentax Olympus Yashica etc.) have 49mm filter thread size E49.
The female thread is 55mm (often used by Minolta, Vivitar and many other lens manufacturers). Excellent quality in metal with black finish, very tough and durable.