determine which idler arm I have

No, you can put the 73 parts in anything 68/ later AS LONG as you have a pitman to match the steering box, which may / may not have the later large spline. As to "why" hard to say. Has nothing to do with a charger, those parts are different.

Only reason you cannot put those parts in a 67 is the old style idler arm, which you say has been modified

And the opposite is true. If you have a 73/ later, and have "those parts" and the appropriate steering box, you can also put the older parts into a 73--76

"Maybe that's what they had"