The Mod 23 was always made at the Olin Kodensha (sp?) factory in Japan from what I've seen. The gun wasn't changed as far as small parts AFAIK, some are gauge specific of course.
The last of the Winchester Classic Doubles as they were called to be imported by Olin was in 86 or 87. That's when they decided to drop that line.
A gun made in the 90's may well be one that was mfg by a company that bought that Japanese factory (Kodesha (sp?) after Olin dropped out of the Japanese gun mfg & import biz.
The new company was named 'Classic Doubles International'.
They made the same guns on the same machinery, same work force, ect.
Lasted about 2 or 3 yrs before bankrupcy.
They also made the Winchester 101 previously made at the Kodesha plant. as a Classic Doubles International version.
Perhaps they changed some fixture and thread attachment specs for mfg purposes and if you have one of this era mfg guns, this is what you are running up against.
I don't know what the markings are on these 'Classic Doubles International' guns are but probably shouldn't have any Winchester markings on them. (Unless a few made with left over parts made it out with them, they'll try anything).
The Winchester/Olin era Model 23 was from 1978 to 1987/88.
They are marked 'Made in Japan on the right side of the right bbl sometimes hidden by the forend wood.
The left side of the left bbl has Winchester marking and an East Alton, Il address.
Standard Winchester proof marks on the bbl and action flats.
The first couple of years the guns were the Model 23 XTR
After 84 they started calling the entire line the Winchester Classic Doubles but the guns were still individually Model 23's with different attachments like Golden Quail, Light Duck, LiteWeight, ect.
Hope you get 'her fixed up and back running again. They are very nice shotguns and handle great.
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FWIW,,when Classic Doubles International went belly up,,the owner decided to take one last plunge and tried again to resurrect the Winchester Model 23 and 101.
He started yet another company this time in the USA named Connecticut Valley Classics.
Frames and forend irons were stainless on these versions and all screws were US spec threads. Again, guns were a copy of the Model 23 and the Model 101.
This venture failed by the late 90's sometime.