I am hoping that 20"is a typo - more likely made with 26 or 28".
IF the DU rep is correct, then you should be able to compare DU guns to non-DU guns to determine that. You might be comparing Brownings or Remingtons instead of Bettinsoli, but the analogous relationship should be there. I have only see the DU (or any other commemorative gun) worth more to someone for whom that particular provenance meant something
“I know that you believe you understand what you think I said, but I'm not sure you realize that what you heard is not what I meant.” - Robert McCloskey