That Shelby frame was used from 39-41.
I'd say that one is likely a 39. It has the earlier long spring seat and the flat fender braces. With Shelby it's sometimes hard to tell. I've had seen 1940 bikes with the flat braces still. But late 39 bikes with the channelled braces.
You can post the serial number, can't always tell much from it tho. I have recorded a bunch of these frame styles from this 39-40 period.
Nice find. I found one with a Western Flyer badge and Shock-ease fork in the Statesman classifieds when I lived in Boise in 95.