RandyS and I are probably the guys here with the most 1920 & 20/26 experience. I don't know about RandyS, but I've been collecting the 1920s & 20/26s for over 25 years, have owned around 100 of them, and have researched them extensively.
Based on the information that you've posted, I have tried to point out where your rifle's attributes are out of sync with the cataloged specs. Based on your description, there are several non-cataloged attributes, so I'd have to say that it is NOT original.
As of 02/04/10, there is 1 1920 and 2 20/26 rifles listed on GB. None of these rifles are original, as they all have been modified from the "correct" factory specs.
If you want to send some clear pictures to me, at remguy260
msn.com, I can offer to provide a more informed opinion as to the originality of your rifle. Without being able to see the rifle in person, or via a series of clear and detailed pictures, I don't know that I can offer any additional input.
Jeff