Sorry, but it's been discussed to death and proven several times that the scoop you describe was never available on Dusters from the factory. Only '72 Demons & '73 and(?) '74 Dart Sports. But they look so good that many people added them afterwards.
Gotta be careful with sealing scoops as well. Some designs can cause turbulence and reduce performance. It may be a combination of scoops style and air pan design. But a friend added a factory style scoop to his '64 Plymouth and had inconsistencies he couldn't troubleshoot. When he partially unsealed the air pan, the problems went away.
