These black key plates belong to the PRO 8 MM series from STERK (Class 4 EN1906:2012). The rosettes have a Ø 51.5mm and are 8mm thick.
They can be combined with, among other things, the 1740 and 1741 Pro 8mm series.
These black key rosettes are made of stainless steel and come in pairs including bottom rosettes and patent screws.
The supplied patent screws are suitable for 38-43mm doors. Thicker doors? Please state the door thickness in comments.