I was looking for information on the plate number. 21 was used on the 2004 A and I didn't see it for any other year. I tried
No, it's got IGWT, so it can't be earlier than 1963. And the last series using the "classic" designs was 1995, not 1999. The low plate number also eliminates most series where the plates didn't reset to #1; pretty much anything from the '70s is out, I think. Still leaves an awful lot of options, though. Edited to add: Oh, I'm a bit late, aren't I?
If I showed a picture of the front, with blotted out year and signatures, you would still be able to tell it was 1963 or '63A because of the Latin lettering on the treasury seal. It says "Thesaur, Amer, Septent, Sigil, while the new 1969 and later $20s say "Department of the Treasury".
I would have guessed Series 1934C due to the appearance of the NEW BACK design with changes to: 1. the title change from "White House" to "The White House" and 2. the shrubbery is taller than previous and 3. a balcony was added to the White House Just my thoughts.
