Here is an article I wrote for a local newsletter, perhaps it will help. Check out all your Shimano components as you said some have been repleaced.
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When were your Shimano components manufactured? Most of them have a 2 alphabetic character date “code”. It is usually located on the non-visible part, e.g. inside of the crank(arm), frame side of the front derailleur, etc. It is stamped away from the model number. Just two letters by themselves. Sometimes enclosed in a circle.

So, once you have the code, what does it mean?
The first letter is the year The second letter is the month
1976 = A A = January
1977 = B B = February
1978 = C C = March
1979 = D D = April
1980 = E E = May
1981 = F F = June
1982 = G G = July
1983 = H H = August
1984 = I I = September
1985 = J J = October
1986 = K K = November
1987 = L L = December
1988 = M
1989 = N
1990 = O
1991 = P
1992 = Q
1993 = R
1994 = S
1995 = T
1996 = U
1997 = V
1998 = W
1999 = X
2000 = Y
2001 = Z
2002 = A
2003 = B
2004 = C
2005 = D
2006 = E
etc.