Filler neck help!

The 73 Dart Swinger I bought has a screw on style(Amazon) fuel cap (see pic). The cap is stuck and won’t open. I have the original locking gas cap and key. I would like to polish that up and use it (see pic). My problem is the filler neck is just a hollow tube and there’s nowhere that it would lock to be able to use it??(see pic). Has the wrong filler tube been put into this car or something? Very confused. Any ideas? I’ve searched online and can’t seem to find the correct filler tube to buy to just replace this one. Or is there perhaps a trim ring that should go on that the cap locks on to?
RB

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