These middle layers are cut for the Reviung39. The top middle layer is positioned between the switch plate and PCB. The bottom middle layer is positioned between the PCB and bottom plate. Both plates are cut from frosted acrylic.
When using the middle layers with the MCU covers, it is recommended to attach the MCU cover directly to the switch plate prior to assembling the keyboard.
Keyboard in photo is for demonstration purposes. This product will only include the middle layers.