The Nikon wide T2 adapter has a M48 thread that is ideal for full frame cameras
The standard T2 ring uses a M42 thread that is suitable only for Nikon cameras with APS sensor. Full frame sensors have a diagonal of 43.3mm which which is wider than the 42mm thread of standard T2 ring. Using a T2 ring on full frame cameras will cause vignetting.
This adapter suits the latest Nikon cameras.