This thread is why I thought it was N frame:
http://www.thehighroad.org/showthread.php?t=356710 (sticky)
See posting by SaxonPig:
Please note that this list is a general guide and not meant to be exact. There is some dispute regarding the dates on some serial numbers and your gun may actually be a year off from what is listed. The precise shipping date as "lettered" can be several years off depending on model. For the exact date on your gun request the letter from S&W Historian Roy Jinks.
Pre-War N frame.
Year/Beginning Serial
1908….. 1-------------1926….. 25000
1909….. 2050----------1927….. 28500
1910….. 5000----------1928….. 29500
1911….. 7050----------1929….. 30000
1912….. 9100----------1930….. 34000
1913….. 11150---------1931….. 36000
1914….. 13200---------1932….. 38375
1915….. 15250---------1933….. 41200 My S/N Would fall here. No letter in my S/N.
1916….. 15500---------1934….. 43350
1917….. 16000---------1935….. 45500
1918-1919 None--------1936….. 47200
1920….. 16200---------1937….. 48700
1921….. 16300---------1938….. 52000
1922….. 18400---------1939….. 57200
1923….. 19600---------1940….. 59000
1924….. 20800---------1941….. 62350
1925….. 22000---------1942-1945 None