I know a lot of guys --- with a lot of shotguns ----but nobody with a Thomson Center shotgun ...and hardly anyone with a Winchester shotgun with the old Win Choke systems in them ..../ and that's part of the reason you aren't getting much response....
A couple of ideas...
a. find a retailer that handles thompson center firearms ...so you can look at the choke ..../ maybe they'll have an older Winchester ...and you can compare the chokes. I don't know where you'll find Thompson Center shotguns ...or who is selling them...so can't help you there.
b. the Winchester X choke tubes you mention - are the old Win Choke - a few years ago when Browning purchased Winchester ...all of the new Winchesters now use Brownings Invector Plus choke tubes. So the WinChokes are not being used in current production Winchester shotguns....
c. talk to some of the used gun stores in your area ....maybe one of them has a Thompson Center and an older style Winchester ...and maybe they'll know if they're the same ...or they'll let you check them / but looking for an old 20ga might be tough.
I would take the Thompson Center barrel with you ...
I think its likely that if the old Winchester X choke ( Win Choke) was reportedly fitting the 12ga Thompson Center ...then its probable that its the same with the 20ga ..../but companies do change threads, etc in their choke systems from time to time.
carrying your barrel to a gunshow is a good idea .../but I think its a needle in a haystack to find a choke that will fit it ....there weren't that many guns made with the WinChokes in them .../they weren't around that long ...and you're looking for a 20ga on top of that ...
It may just come down to trial and error ...or just buying them from Thompson Center...
Briley does have a line of Win Chokes....so maybe call Briley and ask them if they have any info on Thompson Center shotguns ( they're a divison of Smith & Wesson, I think ) ....
http://www.briley.com/winchesterwinchokechokes.aspx
and good luck ....