T-slot nuts for the Prusa XL aluminum profile in sizes M3, M4 and M5. These can be used, for example, to mount display protectors like this one (https://www.printables.com/de/model/698755-prusa-xl-lcd-protector) if you don't have any metal ones to hand. 

If you want the original metal ones, you can get them here: https://www.prusa3d.com/de/produkt/psu-holder-m3ne-2/

Happy printing…