The Cobalt Series Dummy Plug protects the Cobalt Series Bulkhead Connector from water ingress or contamination when a cable is not plugged into it. To retain the Cobalt Series Dummy Plug in the bulkhead connector, you must use a Cobalt Locking Sleeve. Made from black acetal, rated to 600m depth. Includes O-ring and stainless steel retaining ring. The retaining ring is a new feature introduced in June 2022. It can be used to keep the dummy plug and the locking sleeve (sold separately) together. SKU: COB-1002 Quick Guide to Cobalt Connectors　→　https://www.bluetrailengineering.com/post/a-quick-guide-to-cobalt-connectors-with-lots-of-pictures Need CAD files? Please see the Resources page.　→　https://www.bluetrailengineering.com/resources