Power Brake Booster to Firewall Seal

Where the brake booster bolts to the firewall, there is an oddly shaped hole inside the 4 stud cricle. The booster brackets mount to these 4 studs. What actually seals the hole in the firewall where the brake pedal rod comes through? I have the gasket from DMT, and it matches the pattern of the studs and the hole, but there is nothing for it to seal against.
Am I missing a part somewhere? Service manual shows a "plate", but doesn't go into detail.
Any help appreciated.
69 Dart. 10" power drum brakes

George