Hello Russ: What you describe sounds like the old Cutts compensator, which became a factory option in 1939.
Yes used barrels are out there. you have to remember when these guns where in production the norm for a field gun was a 30in. Full choke barrel. About 60-70% of all field grade Md 12's came with one.
About 20-25% came with a modified choke, the Imp/Cyl choke falls into the leass than 10% category along with the earlier cylinder choke.
A plain barrel Cylinder or Imp/cyl will run betewwen $140-$225 approximately, modifieds usually go for $125-$175. It will have to be fitted to the receiver if it has the receiver extention and adjusting sleeve. Another $100-$150 from a gunsmith who has the experience to do that.
To do just a barrel swap, using the current receiver extention and adjusting sleeve sleeve about the same $125-$150.
I'd say you would be looking at a minimum about $300 either way, possibly a lot more, if you have to send the gun off to a "Winchester" gunsmith.
Right now you can pick up a used Md 12 plain barrel shotgun with a modified choke for $300-$500, one with an Imp/Cyl or cyl choke for around $350-$550 depending on vintage and condition.
IMO you would be way ahead to just look for another Md 12.
Regards Dave