that's about what i was guessing. i'm just trying to find out so i can order a new set of millett rear sights. the people who work there sure aren't very helpful.
According to Supica's book, they don't show an "ACY" prefix. These were special order numbers and had only "4" numbers after the prefix. Yours has "5". The prefix letters in the reference book go from ABL, To ADE, no ACY.
The date is 1983.