System 6.0.6 has compatibility problems with the new Macintosh computers.
System 6.0.6 was pulled from production and replaced by System 6.0.7. There
are a few versions of System 6.0.6 floating around that were sent out
before the software was pulled from production.
You should use System 6.0.5 (or System 7) on most Macintosh computers made
before the Macintosh Classic, Macintosh LC, and Macintosh IIsi. System
6.0.7 has features in it that only benefit newer Macintosh computers. This
does not mean that you can't use System 6.0.7 on older products, it just
means that it is not the recommended system.