My PowerBook G3 Series (Bronze keybard) came with Mac OS 8.6 pre-installed but I'm able to NetBoot with the default Mac OS 8.5.1 NetBoot HD.img.

The NetBoot Server Administrator should additionally install the Battery Module Control Strip module and the ATI Resource Manager extension contained in the Important Extra System Software folder on the PowerBook Install CD, the contents of which are shown below.

The third component, the Modem Sound TuneUp extension, should only be installed on the image when PowerBook G3 Series computers are intended to be the only NetBoot clients. The Modem Sound TuneUp extension should not otherwise be installed as it is not appropriate for use with the modems in iMacs and Power Macintosh G3s (Blue & White).

While Mac OS 8.5.1 may boot a PowerBook G3 Series (Bronze keyboard), it does not represent a supported configuration. Mac OS 8.6 and its corresponding Mac OS ROM file were developed in part to support certain hardware features of the PowerBook G3 Series and, as such, is the minimum supported version of the Mac OS on this computer.