Sharing this for those of you who might get caught by it, and making a suggestion for future PCB designs…
After much consternation, I discovered that if you use #4 hardware to mount your BBBW you may have an issue with the board not booting up.
Re: Hole: MGT2: I found that if you use a #4 screw and washer (what I consider a fairly typical / minimal set of hardware for mounting any PCB) that in some cases the washer will land such that it will short out the pins on the SD card socket. The lines that are affected are SD.D2, SD.D3, and SD.CMD.
I would suggest leaving just a little bit more room between the center of any mounting hole and components. For #4 mounting hardware I traditionally leave a 0.350" diameter area centered on the mounting hole and I’ve never had any issues. I realize that some may consider this excessive but if you look up the data in a Machinery’s Handbook and considering tolerance stack-up, you can get an idea of where it is safe to place components.
I hope this saves someone hours of time as I had an assembly that would not boot and there was no obvious reason why. With the BBBW laying out in the open it would work fine, not mounted together with a CAPE board and a chassis it would not work. Obviously, shorting the SD lines to ground with a washer can cause all manner trouble.