Apple's System Software numbering

                                System     Finder   MultiFinder      LaserWriter
System Software Release         Version    Version  Version            Version
-----------------------         -------    -------  -----------       -------
Mac System Software                1.0        1.0       N/A             N/A
Mac System Software (0.1)          2.0        1.1g      N/A             N/A
Mac System Software (0.3 & 0.5)    2.0        4.1       N/A             N/A
Mac System Software (0.7)          3.0        5.1       N/A             N/A
System Software 1.0                3.1        5.2       N/A             1.1
System Software 1.1                3.2        5.3       N/A             3.1  
System Software 2.0                3.3        5.4       N/A             3.1  
System Software 2.0.1              4.0        5.4       N/A             3.3
System Software 2.0.1              4.1        5.5       N/A             4.0
System Software 5.0                4.2        6.0       1.0             5.0
System Software 5.1                4.3        6.0       1.0             5.1
System Software 6.0                6.0        6.1       6.0             5.2
System Software 6.0.1              6.0.1      6.1.1     6.0.1           5.2
System Software 6.0.2              6.0.2      6.1       6.0.1           5.2
System Software 6.0.3              6.0.3      6.1       6.0.3           5.2
System Software 6.0.4              6.0.4      6.1.4     6.0.4           5.2
System Software 6.0.5              6.0.5      6.1.5     6.0.5           5.2
System Software 6.0.6              6.0.6      6.1.6     6.0.6           5.2
System Software 6.0.7              6.0.7      6.1.7     6.0.7           5.2
System Software 6.0.8              6.0.8      6.1.8     6.0.8           7.0
System Software 7.0                7.0        7.0       *    
                     7.0
System Software 7.0.1              7.0.1      7.0.1 
                                       7.0
System Software 7.0.1P             7.0.1P     7.0.1P 
                                     7.0
System Software 7.1             
       7.1           7.1                                          7.1.2
System Software 7.1P**             7.1P    
     7.1P                                         7.1.2
System Software 7.1.1 (Pro)        7.1.1      
 7.1.3                                      7.1.2
System Software 7.1.1            
   7.1.1       7.1.3                                      7.1.2
System Software 7.1.2            
   7.1.2       7.1.4                                      7.1.2
System Software 7.1.2P             7.1.2     
  7.1.4                                      7.1.2
System Software 7.5           
         7.5            7.1.4                                     7.2 & 8.1.1
System Software 7.5.1        
       7.5            7.5.1                                      8.2
System Software 7.5.2          
     7.5.2        7.5.3                                     8.2.2
System Software 7.5.3          
     7.5.3        7.5.5                                     8.3.3 & 8.3.4
System Software 7.5.5 7.5.5
System Software 7.6 7.6
System Software 7.6.1 7.6.1
System Software 8.0 8.0
System Software 8.1 8.1
System Software 8.5 8.5
System Software 8.6 8.6
System Software 9.0 9.0 9.0
System Software 9.0.4 9.0.4 9.0
System Software 9.1 9.1 9.1
System Software 9.2 9.2 9.2
System Software 9.2.1 9.2.1 9.2

System Software 9.2.2 9.2.2 9.2

** Mac OS X is not included on this listing, as it does not follow the System numbering scheme **

Macintosh: System Software Version History via Apple Support

sources: http://www.mac512.com/macwebpages/systemvn.htm