What are these lines?

No, that is the only vent to the fuel tank. Early "non evap" emissions cars were all vented via a 1/4" line welded into the fuel filler neck near the cap, inside the trunk. This line went up in an upside down "U" to the top of the trunk and doubled back down, through the trunk gasket, and open ended in the left frame rail, IE gas could drip in there if overfilled

By plugging that line, you have no vent, as the cap is either not vented at all or is a "pressure vacuum" deal used on all later emission cars

It's in the back of the shop manual, and since you have a '72, you can download one fer free here:

http://www.forabodiesonly.com/mopar/showthread.php?t=132309

der linkeypoo

[ame="http://www.abodyjoe.com/pictures/Misc.%20car%20info/1972%20Plymouth%20Chassis%20Serv%20Man.pdf"]http://www.abodyjoe.com/pictures/Mis...Serv%20Man.pdf[/ame]

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and this thread:

http://www.forabodiesonly.com/mopar/showthread.php?p=1969591363