CD - I think your machine is Foley & Williams, not a National. They take the same shuttle. It looks exactly like my F&W. Measure the width of the throat plate, the F&W is about 1.07" wide and the National is about 1.175 wide. National also has the serial number stamped on the front under the slide plate, the other does not.
The other difference is the distance between presser bar and needle. National has .50" and F&W has .425"