I don't have the answer you need, but my sugestion would be to call and talk to a gunsmith who specialized in the old Winchesters. Those critters have gotten so valuable lately, it's crazy! I'd give Doug Turnbill a call...have the serial # handy, so he can tell if it's an older, blackpowder version, or the smokeless version.
You might have a good candidate for a rebore job...something to consider.
Good luck, and please post what you find out.
Mark