This is a common question. Although I don't know the answer, I seem to remember reading about the three letters in the serial indicating when it was manufactured. Also, on the Taurus web-site home pge, under Customer Services, they list "Find My Model". Maybe you can get info there. You may have to call them on the telephone.
Also, if it is marked "Millennium" it is older than if marked "Millennium Pro". Good luck.