This archived discussion was the only reference I could find regarding the use of a short Apple wired aluminum keyboard A1242—such as the one that came with some iMacs some five years ago—on a Power PC under Tiger 10.4.11, specifically my G5 Quad. I just came into possession of one.
The issue is exactly the same as described by the OP and BDAqua in that some F keys do not work as expected. The only one that I would love to have fully functional is F10 for muting and unmuting the speaker.
Other than that, the rest of the keyboard works perfectly, and I'm ecstatic to have gotten ride of the by now—in my retirement—utterly useless numeric keypad and to have the mouse and the tablet, when in use, within easy reach.
My reason for posting now is just to see if anyone else has confirmed BDAqua's speculation that it's simply a PPC/Tiger issue, rather than a defective keyboard. The keyboard works fine when connected to my Snow Leopard 10.6.8 system through my USB switch box. So the keyboard is not at fault.
I haven't yet tried booting the G5 Quad into Leopard (alternative boot volume available and ready) because, frankly, I have no use for Leopard any more now that I'm hooked up to a Snow Leopard system through a rescued-from-the-trash MacIntel Mac Book.
Thanks in advance for any input from anybody.