Floating Closure together with the Retaining Ring (not included) is used as a forward open closure for all 38mm Aerotech Hardware.
This closure is also a part of 38 Reload Adapter System (38RAS) and you do not need it separately if you have the 38RAS or plan to buy it.
NOTE: The single-piece 38mm Open Closure is discontinued and you need 38FFC (this product) and 38FCRR (available separately) instead of the discontinued forward closure.