Emissions Control System ID help, 1971 Duster 340

PlymCrazy said

Now I’m confused. A non-vented cap I’d think goes on an ECS car. The way I’m understanding ECS at this point is that it DOES NOT allow for gas vapor to vent to atmosphere but rather sends the unused vapor back into the fuel system to be burned. Please set me straight here.
- Nick

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I got caught up in improper terms. NONE of the pre-70 or the 70 without evap was vented. It is an obvious, solid cap when you look at it, and the vent is a tube welded into the high end of the filler neck inside the trunk, and then leads down and exits through the trunk floor grommet

None of the evap controlled cars are "truly" vented. That is the caps are not intended to operate as a vent. They were / are known as "pressure vacuum" caps because they have spring loaded pressure relief valves to let in or let out pressure after some build-up. The operational vent in those cars is provided by the tube running up to the engine bay.

If you have not done so I really again suggest you download the 69---about 73 manuals there from MyMopar and read the fuel and emissions sections.