I have also had this problem on my PBG3. Here's the basic info:
1. It is a legit original.
2. It installed on my G4 at work just fine.
3. It is still bootable on the G4.
4. My PBG3 is supported (it's a 300MHz Wallstreet)
5. It is not bootable on my PBG3 through System Disk.