Effective Spring 2016, to improve the sealing performance and O-ring life span in Shearwater NERD and Petrel2 devices (including DiveCAN controllers), the battery compartment O-ring gland design has been changed from a crushed gland to a radial gland. The new radial gland caps can NOT be installed on older computer cases, i.e. those manufactured prior to Spring 2016. Identification of the new radial cap can be made by observing the differences in the table image above. The cases which take the new cap will be engraved RC under the strap.

Just to be clear, if you use the wrong cap, your dive computer will flood and it will not be covered by any warranty. Battery caps have no need for periodic replacement, you should only need a battery cap if you lose yours.