I managed to find this list so it looks like it started appearing with version 2004:
18.0 (LMS Tech) = AutoCAD 2010
17.2 (LMS Tech) = AutoCAD 2009
17.1 (LMS Tech) = AutoCAD 2008
17.0 (LMS Tech) = AutoCAD 2007
16.2 (LMS Tech) = AutoCAD 2006
16.1 (LMS Tech) = AutoCAD 2005
16.0 (LMS Tech) = AutoCAD 2004
15.2 = AutoCAD 2002
15.1 = AutoCAD 2000i
15.0 = AutoCAD 2000
14.0 = AutoCAD R14
Which leads to another question. What is the 's' after the number in some versions?