Nikon BR2A reversing ring
The Nikon BR2A is a ring for reversing a lens onto a camera body, or (more frequently) onto accessories for close-up and macro photography, like extension rings and bellows. This type of reversing ring has a male bayonet on one side and a male thread mount on the other. Do not confuse this accessory with rings used to reverse one lens and attach it in front of another lens.
The Nikon BR2A ring shown above is a solid brass construction, and is unique in the fact that the bayonet can be mounted on a camera body in three different positions, roughly at a 120 degree angle from each other. The ring has three dot markings on its rim to facilitate mounting onto a camera or accessory. This lets a user choose the best position for allowing the controls and scales of a reversed lens to be accessible. The BR2A has a male thread for screwing into a 52 mm filter thread at the filter mount of a lens. If you intend to use a lens with a mount for a different filter size, you must use an adapter ring.
There are third-party reversing rings with aluminium bayonets that do not look or feel very durable. Anodized bayonets are especially a bad idea, because their surface coating can come off as aluminium oxide dust when mounted on a camera body, and these particles are very hard and easily scratch optical surfaces.
The BR2A supersedes the old BR2 model. The BR2 apparently had a problem with the triple slot in the mount (functional in allowing the ring to be mounted at three possible angles) trapping the head of the "screwdriver" on the camera body (used for mechanical autofocus) and preventing it from turning, thus damaging the autofocus motor and gears. Therefore, Nikon states that only the BR2A should be used. While you could use a BR2 and avoid damage by setting manual focus on the camera body, there is a real risk that you may forget to do this, causing damage far in excess of the cost of a BR2A.
The above explanation of the incompatibility of the BR2 with modern cameras does not come directly from Nikon, and has been gleaned from threads on photography sites. Nikon does not explain the reasons why you should avoid using the old BR2 model.
The BR2A can be seen in use here and here. It is probably the most versatile source of a high-quality Nikon F male bayonet, and is easy to use in building special equipment without the need for drilling and machining, by combining together lens adapters and extension tubes.