Per Douglas Murray's book "The Eighty-Eight," serial no's 004 - 35999 were made during 1955, the first year of manufacture. Mine is 21XXX and works well. Be sure to use a snap cap, as the firing pins at first were pretty brittle. I had mine break 12 years ago and had the dickens of a time finding a replacement. Finally found a man (in California, IIRC) who had bought all the spare parts when Winchester changed hands. He said that there were three different firing pins made which all will work, but the newest version was the best.
-WH-