This is a 3D printed case for the 4-Player Adapter from Individual Computers.
The 4-Player adapter (sold by Individual Computers, https://icomp.de/shop-icomp/en/produkt-details/product/id-4-player-adapter.html#filter=* ) plugs to the user port of the C64, C128, VIC-20 or CBM610 computers. It was originally used by the Commodore 64 game "Bombmania", but has now turned into the de-facto standard of 4-player solutions for the Commodore 8-bit line. It is supported by the following games:
Alone in the Green
Amazing Maze
Bombmania
Garrison
Hockey Mania
IK+ Gold
Marble Madness
M.U.L.E.
Phong!
Quadris
Quadtron
Rampage Gold
Snacks 4 Snakes
Space Lords
Square Attack
Ivan 'Ironman' Stewart's Super Off Road
Tanks 3000
Team Patrol
Tour de France 2005
Although mechanically compatible, this adapter does not work on the Commodore plus/4.
Two M2 heatsets and M2/6mm cap screws are required for the assembly of this case. They can be obtained here:
http://corei64.com/store/products.php?46&cPath=12