Not sure of the exact year, but definitely pre-43 DX, so 41 is a good guess. 56-link skiptooth chain would be right, you might get away with a 55-link. It all depends on how many teeth are on the rear cog. That part I cannot see.
The '40 DX had a plain looking guard, but the '41 used the "bird wing" that attaches to the seat stay at the back and has a clamp up front. Much nicer and sleeker looking guard. I'd post a pic for you but they screwed up photobucket so bad I can't even figure out how to use it anymore. :x
Schwinn also made chrome repops of the birdwing that are really nice. Gary