It's a faceplate for Adapter 9 from 2040, it's mounted on top in order to give it a slightly more attractive finish. Installed on the main PCB using screws and M3 nylon hex standoffs. As it's just a piece of decorative flair, check out the related shared projects in case you need either the main board, or a even a completely different version of the faceplate.

NB! Faceplate B has a cutout for a specific version of a commonly available 0.96" 128x64 I2C OLED display. Unfortunately as there are different versions of it and no real product numbers, here's what you'll be looking for. This is known as the "New"-version, the old one is usually listed as the "Original" version. When looking for the correct one, the older one has a wider ribbon shown at the bottom. Beyond these two, there's an even older version that's easily recognized by a PCB that's wider than the glass.