This might shed some light on the alphabet/year mapping:
http://shiningbrightly.blog.so-net.ne.jp/2009-12-01
1984 = N
1985 = I
1986 = H
1987 = O
1988 = R
1989 = E
1990 = C
1991 = D
Supposedly it follows the order of the unique characters in "NIHON RECORD", switching to regular numbers after 1991.