[Phonograph]

Originally Posted by Gigablah

...

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.

actually it goes till 1992 = K
for NIHOn RECorD Kyoukai (日本レコード協会)