well first off its NOT made by Winchester?
a model 870 is made =by Remington
so, if you been looking for a Winchester 870 barrel, odds are you didn;t find any!

next is, you might want to look up the serial number and see what exact model gun you have
as some guns based mostly on age , might not fit teh same as every barrel, as I believe there were some slight changes from the very first 870's to the newer one's!

another option can simply be, send the barrel out and have it fitted for screw in chokes
Briley does this, ( https://www.briley.com/ )
as well as other good gun smiths!
I had them do this to a few rem shotguns over the yrs as well as other brands, you will need to measure barrel to make sure there is enough meat on things to do so, but it can also be a option !

they can also cut back a barrel and add screw in chokes too, if your barrel is longer than you want (lots of older guns had very long barrels , 30-28 inch and likes)