Reply To: Can anyone Identify this OMC piece

[frankr]

Changes in laws made the vented cap illegal because it can emit fuel vapors. Even slop gas out if full. The mystery vent is a one-way vent, allowing air to enter the tank but not escape. It probably also has an over-pressure relief to let air out before it blows up. I suspect somebody has defeated it by installing that hairpin clip on top.