These are basic 3D printed mounts. There is 2 holes sized for M3 hardware that are 90 degrees from one another. It’s called 5x5 because the hole center is 5mm from the opposite hole face.
These are super light and can be used for battery tray mounting chassis’s. Just like the body mounts, these will arrive as a parts tree. Once trimmed some white will be visible and a sharpie will hide it if desired.