This is a mint condition Mamiya 2x Teleconverter 2x N. It has absolutely minimal signs of use – a superb example. The Mamiya instruction leaflet can be downloaded from this website.
This converter is recommended for use with most 645 lens from 150mm upwards, and doubles the focal length of the attached lens for the loss of 2 stops exposure. Your 210mm f4 becomes a 420mm f8. You can even stack two converters together if you wish.
The quality of this Mamiya converter is far far superior to any of the third party converters – Kenko, Rokunar, Vivitar, etc – and if you are serious about a 2x converter this is no doubt the best that there is by a long way.