..he asked for "American" currency.. suggests his destination was outside the US
This point is debatable, Fly. We simply don't know if DB Cooper actually said, "American" currency.
One 302 says that Tina stated that he simply asked for 200 grand "in cash." Other interpretations of what he said also exist, such as Flo's note that he as for "negotiable American currency," in one of her cockpit writings.
As with most evidence of variable certitude, we need to consider all possibilities.
Correct, it is debatable.. almost everything is.. I used the term "suggests" not "confirms"... both the stew noted and pilots noted "American" currency.. it is possible it was added but unlikely.
I have heard the repeated claim that the crew added it but that doesn't really work...
If the the crew were all American why would they add "American" to currency...
the logic doesn't work..
Either Cooper said it or the American crew added it, which is more likely..
Adding "American" to currency suggests a foreign influence.. if Cooper didn't say it then why would an American crew add it. They wouldn't.
Think this through, the arg.. Cooper didn't add it because there was no foreign influence but the American crew with no foreign influence did add it.