[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). For more details on these developments, see my notes on software releases.
The "System Software" numbers used in Apple Knowledge Base article TA31885 are based on a series of CDs that first appeared in May 1989 at the Worldwide Developers' Conference, where Apple Developer Group distributed a "Developer Helper" disc called Phil and Dave's Excellent CD (published in two volumes: a pre-release version for the conference and a later release version), authored by Dogcow Productions (Moof!).* A similar disc came bundled with Apple's quarterly technical journal, develop, which launched in January 1990. Both of these early CDs include and archive of disk images for early system software versions numbered 0.1, 0.3, 0.5, 0.7, 1.0, 1.1, 2.0, and 2.0.1. These disk images are a jumble, full of omissions and repetitions.
The flaws in this archive were never corrected and were passed along on subsequent Apple Developer Group releases throughout the 1990s. For example, the Apple Legacy Recovery CD (May 1998) includes disk images for early "System" versions numbered "System 1.0" and "System 2.0.1". In fact, these are System 3.2 Finder 5.3 and System 4.1 Finder 5.5. Perhaps, if the "0.x" numbering scheme used on the CDs were aligned with the early history of Mac Software, then it could be useful. But it is not aligned. To see why this is a problem, you just have to look at the Knowledge Base.
How did this scheme come about? Apple started using version numbers on Macintosh disks in January 1986. 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 the version number of the named disk. They were also used on the labels of the update disks Apple sent to dealers (an example is shown below), to distinguish between them. Apple Developer Group and/or Dogcow Productions used these disk version numbers as "System Software" version numbers. Looking at the disk version numbers (in parentheses below), you can see how they came to result in the problems on the developer CDs and the errors 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. The producers could have used the System file version numbers as its basis, which would have made for a nice, neat progression leading up to System Software 5.0 in October 1987. It would have also allowed them to easily check for errors among the disk images. When Apple introduced the concept of "System Software" in the System 4.x Read Me documents, the idea can only have been to use the System version numbers to identify earlier releases. It's unfortunate that Apple Developer Group and/or Dogcow Productions didn't understand this, or didn't think it was important.
That said, it's also true that Finder version numbers better reflect the evolution of the software itself. Apple didn't release six major Macintosh operating systems in just over four years! Instead, it was more like three: [1] everything up to Finder 4.1 (MFS), [2] Finder 5.x (HFS, support for hard disks), and [3] Finder 6.x (MultiFinder, support for high-density floppy disks).
* I was not aware of the date of the first Apple Developer Group CD until Alexis Jhon Gaspar alerted me to its existence. You can find it at Macintosh Garden. Previous versions of this page reference the 1990 develop CD, but don't mention 1989's Phil and Dave's Excellent CD. He also persuaded me to provide the original Finder 1.0 disk images, for their singular historical importance, and pointed out useful information about the history of System 3.1 Finder 5.2.