That was just one data point and their expert specifically called out the 51 serial number on the 20" bike as a fake.
I've seen the most of these DD bikes listed and sold on Liveauctioneers.com and some of the finest examples have captured in their listing description, one year only. These were all 20" and 24" bikes. Go take a look, just sign up for free then you can peruse all the results and listing details, including some very healthy sold prices.
I agree with you that the 26" was offered beyond '49. I'm not sure it even came out until late '49 with the completely different fender/head assembly.
I'm really a vintage Scwhinn Stingray guy, always learning, so I'd love to hear more about this, but based on my research and limited info out there, like no Shelby catalogs to download, this is what I believe to be factual, thanks