Japan Mint started to use these marks in 1979.
The following (constantly updated) table includes all currently known time coordinates for these marks.
AC - June 28, 1979
AB - November 28, 1979
BA - February 27, 1980
AJ - April 26, 1980
BB - April 26, 1980
AL - April 29, 1980
AL - May 28...
dark blue ribbon medal of honor
japan mint marks
japanese medal
japanese medal of honor
japanese medal with mark
japanese medals marks
japaneseorder with mark
japaneseorders marks
letter-digit marks on japaneseorders and medals
markedjapanese medal
markedjapaneseorder
two-letter marks on japaneseorders and medals