The "Book" says a 32 D A 4th Model was mfg from 1883 to 1909, in the S/N range of 43406 to 282,999. Your # 55XXX would have been made in probably 1884 or 1885. They Mfg. roughly 239600 over 1883 to 1909, about 26 years, or about 9215 p/yr. Keeping in mind that S&W did not ship in strict S/N order, it could have been in the vault several years before shipped. Since the cut off date for Antiques is 1898, I would think you are very safe in this qualifying as an Antique..