Keep this in mind. Anything today can be duplicated, made to look, feel and smell like a real Winchester. Heck, we have an engraver above with the talent (and experience) to perfectly reproduce any factory work. If the letter isn’t in Cody, there is no providence. Providence drives price. As Silvers suggests, if it fits well, shoots well and you want it, buy it. Only thing I would suggest is price it against the current Galazan price list to duplicate, take twenty percent or more off and that should help establish a fair price.
Without a letter from Cody proving its origins, be careful not to get yourself buried in it, it doesn’t matter how or who put it together, it is what it is.
If I ever had an issue with the 28 gauge model 21, it was the weight in the barrels.