Please post the number when you can. Unfortunately there is no serial number list for Shelbys. Dating is usually best done by the paint and what parts were/are on the bike. In your case, the bike has the crank and chainguard that appeared in 1940. The paint details match other 1940s also. The chainguard is the longer version of the two in that style and paint that Shelby used. The shorter version is shown in the 40 catalog. The longer version may be from 41. I also think that frame was used in 40/41 and wasn't available in 39. And the fork is also seen in 40/41 and doesn't have the type of crown seen in 39.