It is flat. There are some nice aluminized ones for some chrysler v6 that fit my 2.25 catback piping real nice that are available from the auto parts. I use them for all 3 joints & the oem donut for header to midpipe connection.
Mine was a disc. But the ones Racing beat initally sent me were the wrong ones. I sent them a fax outlining the old gasket, and they sent me one labels for a 1990 era miata. It fit, but wtf.
Did you ever find a place online to buy these? I have an exhaust leak and I'm guessing it is one of these gaskets. That or that crush gasket/washer thingy in between the header and midpipe. My car sounds hilariously loud now, especially at idle.