Was replacing the thermostats in my motors, and when I pulled the "stock" gasket out of the package (Sierra # 18-3644), I noticed that it wasnt even close to the one that was in there. The one that was on the housing was black..had two little brass rivet looking things on each side by the bolt holes..the inner diameter was larger..and was almost twice as thick as the new gasket. Any ideas ? Thanks guys.

Get a Merc gasket. Without the rivets, your temp guage sender will not ground properly and will not work. Do not use sealer on the merc gasket.