Well, the M66-2 was made from 1982 into 1986. When looking through the three letter prefixes in those years, I don't see "AUY" at all; I do see "AUU" in 1986, and "AUV" in 1987. The three letter codes generally run from A-Z (AAA, AAB, AAC..... AUV, AUW, AUX, AUY,... etc.), but not always and there are many out of sequence pairings. I even found an "ASR" in 2004, when almost all of the codes in that year start with "C". There are many of those 3-letter possibilities I haven't seen listed, but since people have guns with them, they must exist. My guess would be sometime in 1986, but that's a SWAG.
How is yours equipped? (barrel length, round or square butt, etc.) Let's see some pictures.