A 54mm diameter BROLITE concealed fixed escutcheon. A base plate is fixed to the door first and the cover cleverly attaches to the base with a twist action (similar to a bayonet light bulb). 
This is a more modern size and look that is particularly popular in Europe. Available in Walnut or Black Bakelite. 
Available to suit a Euro-Profile cylinder as well as a standard key of a mortice lock.
We also produce a face fixing version of this.
Measurements:
Diameter: 54mm
Projection: 7mm