This is a cover for the DivIDE-AY device that is used on the Sinclair Spectrum computer. The cover slides over the DivIDE to provide a little more protection for it. You do need to remove the cover to access the DIP switches and jumpers. There is a hole beside one of the DB9 connectors where you can add a screw to hold the cover in place. (I don't bother with the screw). If you do use a screw, make sure that it isn't too long. You don't want it damaging the circuit board below.

The 3mf file contains several pieces that need to be "subtracted" from the model before slicing. They are provided so you can make adjustments to the hole sizes and placements. The STL file is ready to slice.

This should be printed on its face. You will need some supports.