Sounds like a "Step-Down Ring" - you can get them in various 'strengths'.... the '0.45x' means that it turns the main lens into a lens that acts like it's only 45% of its focal length..... e.g. a 28mm lens becomes something like a 12mm. For your Nikon you could have a look on the Nikon websites for 'genuine' accessories, or somewhere like
Bowen's (who make their own) for the right filter size of your lens. You shouldn't need to pay more than about 15 quid. Here's
another site that had them but it's sold out and it's in the USA..... but you get the drift.