[Apple Macintosh before System 7]
The earliest references to "Macintosh System Software" are in the Read Me documents on the System 4.0 Finder 5.4 and System 4.1 Finder 5.5 disks. The first named and numbered release was Macintosh System Software 5.0 (System 4.2 Finder 6.0). The "System Software" numbers used in Apple Knowledge Base article 15582 for earlier releases appear to be based on a CD that came bundled with the second issue (April 1990) of Apple's quarterly technical journal, develop. The CD includes disk images for early "System" versions numbered 0.1, 0.3, 0.5, 0.7, 1.0, 1.1, 2.0, and 2.0.1.* Unfortunately, the disk images on the CD are a jumble of omission and repetition. These mistakes have been continued as recently as the Apple Legacy Recovery CD (May 1998), which includes disk images for early "System" versions numbered 1.0 and 2.0.1. These confusing numbers also appear in the technical specifications Apple provides for its early hardware and elsewhere in the Knowledge Base.
The source of the confusion is simple. Apple started using version numbers for disks around January 1986, as indicated in parentheses below. You can see them by selecting the disk or mounted disk image in the Finder and using File > Get Info to view the comments. They were also used on the labels of the update disks Apple sent to dealers (an example is shown below).
It is clear that the people who put the develop CD together tried to use these disk version numbers as "System Software" version numbers. Looking at the disk version numbers (in parentheses below), it's easy to see how they would come to result in the errors on the CDs and in the Knowledge Base.
System 3.0 Finder 5.1, January 1986:
System 3.1 Finder 5.2, February 1986:
System 3.2 Finder 5.3, June 1986 [disk images]:
System 4.0 Finder 5.4, January 1987:
System 4.1 Finder 5.5, April 1987:
Revised June 1987:
The most unfortunate aspect of this debacle is that a sensible, straightforward approach already existed for the numbering scheme used on the develop CD. It could have, and should have, used the System file version numbers as a basis for its "System Software" numbers. This was clearly the intent of the people who formulated the version number "5.0" for the first named "Macintosh System Software" release in October 1987. Using a third digit for the System 4.1 Finder 5.5 disk revisions (4.1 for the April 1987 set; 4.1.1 for June 1987; and 4.1.2 for August 1987), everything else falls into place.
* Special thanks to Willi Kusche, who gave me access to all of the disk images from the original develop CD.