20A factory rebarrel?

[Deldillco+]

I have a 20A that has what appears to be a 38 barrel and full length button magazine. At first I thought that it had just been updated with the newer button magazine. Then I noticed (how could I be so blind) the dovetail placement of the rear barrel sight was like Models 37 and 38. My next conclusion was that a previous owner had replaced the 20A barrel and magazine with the same from a 38. Further examination of three 38's that I have in inventory showed that each had different barrel markings/patent dates than the one on the 20A, which is marked with just "Patented" and no dates.
Why just patented? Did Marlin at the time have a "common" replacement barrel for 20, 38 and 39, so that "patented" covered all 3 (and maybe others) models?
Just trying to get gray matter juices going on a cold winter night....