These brackets are to be used for mounting an ATX motherboard in a Powermac G5 case without cutting the existing mounting pegs.
A Fusion360 project can be
...Show more dowloaded here :
http://a360.co/2GfSDiA
Thread on tonymacx86 about this build :
https://www.tonymacx86.com/threads/g5-case-mod-keeping-the-case-as-original-as-possible.229001
I'll try to update with pictures when I rebuild the computer.