Mercruiser Alpha One part

[64Amphicar]

I went to replace the water impeller on my 2006 Bayliner 185.
When I separated the upper and lower half of the stern drive this was there.
I have no idea what part it is or where it goes??
Any ideas??

 

[1960 Starflite]

Backflow protection to help keep water out of exhaust upon sudden stops from full speed. Goes in exhaust elbow.