The first 55mm Micro-Nikkor for the Nikon F was a preset lens. The close-down ring was also the filter-mounting ring (52mm Nikon F standard). This meant that any filter would rotate when the lens was stopped down. It also made it difficult to reverse-mount the lens on a bellows. The M-B ring screws into a smaller thread well inside the front of the lens (31mm) which does not rotate when the lens is stopped down. This allows the reverse mounting of the Micro Nikkor on a bellows and still easily stop down the aperture. The second version of the pre-set Micro Nikkor has a projecting filter thread that eliminates the problem. The M-B ring was not a good solution since over-tightening the ring and then trying to remove it could cause the lens to unscrew (oops!).
You have a very rare item. I own one and use it to adapt a telescope eyepiece to any Nikkor lens with a K-3 ring. WES