View attachment 11756
It goes right here. The purpose is to prevent the battery from falling inside the enclosure since the PCB sits a little higher than normal.
The board was originally going to use another method but that part has become hard (impossible) to find, so you'll need to clip four pins from the header and it'll drop right in.
Of course you don't need it if you don't plan to use a battery.
I'll post a pic of my build shortly.