Muffler Gasket for Arrow Slip-On

[CaptainChris_imp]

Last night I finally received my Arrow silp-on. Went to do the install and it was a fairly straightforward process. Afterwards during my sanity check with the service manual, I noticed that the stock muffler has a gasket (part # 18392-MG7-750) that goes between it and the header. I didn't see this gasket at all while I was removing the stock one, so the Arrow went on without it. How important is this gasket, and is it just for the stock muffler or should any aftermarket one use it as well? After closer inspection on the stock muffler there is something that looks like it could have been a gasket left on the connecting surface, but it doesn't look like it wants to come off and has the feel of a hard plastic.