Some additional information I found in the internet...
What is the difference between the BR2 and BR2A Rings?
The BR2-series rings are designed to allow a lens to be reversed and mounted backwards for close-up photography.
The BR2 (older style)ring will interfere with the CPU connections on an autofocus camera and prevent it from mounting properly. The BR2 cannot be used on a digital SLR for this reason. The BR2A is needed to reverse mount a lens to an autofocus camera (including Nikon D-SLR's). The BR2A works with manual cameras as well. Both the BR2 and BR2A mount to a 52mm filter thread.
Nikon 105mm is the best