That wasn't true for all manufacturers.
I know Brunswick's code had the first letter representing the month it was made,
Jan - A, Feb - B, Mar - C, April - D, May - E, June - F, July - G, Aug - H, Sept - I, Oct - J, Nov - K, Dec - L.
and the second letter represented the year.
2005 - T
2006 - Y
2007 - Z
2008 - A
etc.