25-20 is a caliber, not a model number. If the gun is a lever action, it's probably a model 1892, which were made up until WW2. In that caliber, it's probably from the early 1900's. If you go to: http://armscollectors.com/sn/windates.htm select the proper model number and enter the serial number, it will give you the date the gun was made.
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