Historically, Poseidon incorporated the OPV for their Jetstream and Xstream second stages in a unique fitting on the end of their non-standard low-pressure second stage hose. In 2008 Poseidon updated their Xstream first stage design (the Mk3 designation) and Xstream/Jetstream second stages to no longer require a Poseidon-specific low-pressure regulator hose.

There are adapters available which allow use of standard regulator hoses with Poseidon regulators manufactured prior to 2008. However, you MUST include an over-pressure relief valve, either by adding an OPV to an unused LP port on your older (Pre-2008) Xstream first stage - OR - use a hose adapter that incorporates an OPV - OR - use a current (manufactured 2008 and later) Xstream first stage that already incorporates an OPV.