S&W used 2 different codes for this purpose.
Old one, 4 digit was: XYYY - where X was the last number of the year, YYY number of the day in the year (fe: 2123 - 123 day of 2012, means: May 2nd, 2012)
New one is 6 digits: MMDDYY - this is obvious
(fe: 050215 - May 2nd, 2015)