You may or may not be nuts; your wondering about radiator cap construction is insufficient to establish either case as fact.
You're correct, rad caps for recovery systems have an upper seal not present on caps for open-vent systems. A cap for a recovery system has two rubber seals: One on the face of the main valve, which seals against the lower flange ring of the radiator neck, and another larger one just under the cap's underside, which seals against the upper flange ring of the radiator neck. The upper ring's seal against the upper flange means that when the system cools down and coolant volume in the radiator decreases, the resultant suction is channeled via the open centre valve through the overflow tube, which draws coolant from the reserve tank back into the radiator.
