These rings are used on the female connector of a shore power cordset and thread onto the vessel's shore power inlet to make a watertight seal.
Marinco's 100R 30A power cord Threaded Ring (as well as their 500R 50A power cord Threaded Ring) can be used on ALL Marinco inlets of the appropriate amperage, since both new and older inlets have the legacy threads.
The 100R ring is designed to be compatible with other manufacturers' 30A inlets, but can only be guaranteed to fit Marinco's own inlets.
Made of black Lexan®