Well, I called Winchester, and here is what I was told:

The 120 is the same as the 1200 and can take both 3" and 2 3/4" shells.

I was told that if the barrel does not have the capability to have screw in chokes, then steel shot should not be used. Mine does not, so Steel will be out.....but I assume I could still purchase a newer barrel for this gun???? The only reason is steel shot appears to be so much cheaper.

Thank for all your help.