You can contact the Records Office of Cody Firearms Museum at the Buffalo Bill Historical Center. The Cody Firearms Museum holds the records of 3.5 million individual Winchester firearms and many LC Smith and Marlin firearms.
Information that may be in a record includes the date the gun was received into the warehouse, the configuration of the gun, and the date the gun shipped out. The company to whom the gun was sent may be available for very late Model 1873s and 1886s and most of the run of the Model 21. There are no records for the Models 42, 12, 54, or 70.
There is a charge for Factory Letter information but this and any other questions should be answered by contacting the Records Office.
Cody Firearms Museum Records Office
720 Sheridan Avenue
Cody, Wyoming 82414