The Equilibrium is used to weight the back of the pen. Inserted through the end of the barrel, the rings are threaded into the barrel using the end of the tube shown above. You can add up to five rings. They come in a lovely grey suede bag and the black hard rubber tube.
The standard Equilibrium pen, as with all others comes in different finishes. It is also available as a special order in the Long, Portable, and Piccolo size as pictured above.

This is only available through special order.