Now we're getting close! The EH, EO and ER coded vents did have the pat.#, but EI onward did not. I can also confirm that's when the dropped an L from ILL. So anything with a letter or two through 1996 had the pat.#, and anything after that did not.
I think...
And for those keeping score, we still need a D!
A - 1979
B - 1980
C - 1981
D - 1982 - MISSING
E - 1983
F - 1984
G - 1985
H - 1986
J - 1987
K - 1988
L - 1989
M - 1990
N - 1991
O - 1992
P - 1993
EH - 1994
EO - 1995
ER - 1996
EI - 1997
EZ - 1998
EE - 1999
DD - 2000
DA - 2001
DU - 2002
DT - 2003
DH - 2004
DO - 2005
DR - 2006
DI - 2007
DZ - 2008
DE - 2009
AD - 2010
AA - 2011
AU - 2012
AT - 2013