The original crossover pipe for a single exhaust system had the 3-bolt flanges made onto it and the donut seats were built into the pipe on both sides. Passenger's side manfolds had a flat gasket surface that the heat riser valve seated against, and the bottom of the heat riser was recessed for the donut. Some factory original donuts were made of cast iron, rather than the wound asbestos donuts that came in gasket sets. Running without a heat riser will result in really poor engine performance during warmup.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!