Trash: the earliest G5s came from the factory with a special G5-specific build of Mac OS X 10.2, and all subsequent models had some version of 10.3 or 10.4.

A late-model G5 from between early April and late June of 2006 probably came with 10.4.6, as that point release of the OS would've been current during that time frame, and since the last G5 was discontinued in August of that year, that would pretty much include the very last G5s ever produced.

EDIT: By your exclamation, I assume you've already found this out for yourself!

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