System 4.3 (Finder 6.0, System Software 5.1) was mostly a version bump to match a small update to the Finder, released in November 1987. It added a new version of Apple HD SC Setup for updating the drivers on Apple-branded hard drives, and an updated LaserWriter driver.
Besides that, it is for all practical purposes identical to System 4.2 included in System Software 5.0. Some system version lists don't mention to this release, implying that it probably replaced System 4.2 without much hype.
- System 4.3 / Finder 6.0 (System Software Update 5.1), The Mac 512. Archived 2016-12-02.
- Macintosh: System Software Version History at Apple Support (2012-02-19)