The barrel should be 25" at a minimum. You measure a M12 barrel from the front of the ejection port. By 1922--the barrel should have been 26". Early M12 20ga. guns were 25" long. It's possible that there were still 25" barrels in 1922. .020 would be at the very bottom of the full choke range for a 20 bore Winchester. It's more like improved modified. If your barrel is truly 24"-then its been shortened a bit and that could explain the degree of choke left in the barrel.

nid-28