Simple extrusion covers to protect against dust and filament residues. They fit very well and can be easily removed with a flat screwdriver.
Interestingly, I have two different lengths of rear extrusions on my printers. It seems that on the original MK4 kits the length is 115mm and on the older MK3 extrusions the length is 120mm.
The author marked this model as their own original creation.