TP Rocker Box Gaskets.
Rather than use the S&S rocker boxes that come with the S&S engines, Big Dog decided to put Top Performance rocker boxes on their engines. Best I can tell, it is so you can get them out from under the very close frame. It's tricky to replace the gaskets, because the frame it so close. The one O-ring idea that Big Dog uses for these and the primary covers is simple however they often leak and require replacement.