Probably. I do not have a full list with what letter denotes what year, but I do know the 1st letter denotes production year and I can compare this letter with my own frames. With lilolee's Koga I found out that the serial starts with the same letter as the serial of my Team Miyata Herbold Signature, which is from 1991.
Moreover, if your paint is still in tact, you can also simply check the catalogues on the Koga site.
Moreover, if your paint is still in tact, you can also simply check the catalogues on the Koga site.