The serial number likely has a letter prefix or suffix from which indicates year of manufacture.
Prefix
E - 1974
F - 1975
G - 1976
H - 1977
J - 1978
K - 1979
L - 1980
M - 1981
N - 1982
P - 1983
R - 1984
S - 1985
T - 1986
U - 1987
V - 1988
W -1989
X - 1990
Y - 1991
Z - 1992
Suffix
1993 - A
1994 - B
1995 - C
1996 - D
1997 - E
1998 - F
1999 - G
2000 - H
2001 - J
2002 - K
2003 - L
2004 - M
2005 - N
2006 - P
2007 - R
2008 - S
2009 - T
2010 - U
2011 - V
2012 - W
2013 - X
2014 - Y
2015 - Z