As a 20g, it will have a Featherweight frame.
If it's after about 1940 it will likely have a Single Sighting Plane rib with a higher rib extension and action breech

[Linked Image from photos.smugmug.com]

20g barrels are tough to find, and expensive.
If about the same DOM the barrels might just drop in, but should of course be checked by a qualified smith.