Mac OS 7.5.3 Revision 2, otherwise known as System 7.5.3 Revision 2, was a "rapid response update" from Apple to address bug reports after Mac OS 7.5.3 was released.
After Mac OS 7.5.3 was released, Apple Inc. released -- in the months following the 7.5.3 release -- a few minor patches and updates, the sheer insignificance of which did not warrant any of them a version number. However, the bug reports soon came pouring in. Apple had to address errors on PCI-based Power Macs and PowerPC PowerBooks, and thse necessitated larger updates.
Apple was in no mood to drag its feet and announced its "rapid response updates". This set the stage for Mac OS 7.5.3 Revision 2. (Although a Revision 1 never actually occurred, the number 2 was chosen to reflect the fact that earlier Revisions -- those that never made it to end-users -- did actually exist.)
The two-disk upgrade fixed problems on the PowerBook 5300 and PowerBook Duo 2300, as well as any PowerBooks, with or without PowerPC chips, that ran Connectix RAM Doubler; any PowerBook 200 or 500 models which were upgraded to PowerPC status, as well as all Power Macintosh 7200, 7500, 7600, 8500 and 9500 systems.
In the About This Macintosh dialogue box, the Revision modified the name of the OS to read "System 7.5.3 Updated to Revision 2". However, on Macintosh computers that came with Revision 2 already installed, the OS was named, instead, "System 7.5.3 Revision 2". Despite the minor changes in the naming game, the two operating systems were exactly the same.
The very naming got even more confusing for people. Mac OS 7.5.3 was installed by System 7.5 Update 2.0, but this update was called System 7.5.3 Revision 2 -- they mistook the Update 2.0 and Revision 2.
Despite the numbering confusion, the revision finally re-established a standard system for all Macs.