This, believe it or not, is the reflecting mirror from a German WWII searchlight.
The Channel Islands were the only part of the United Kingdom to be occupied by the Nazis during the Second World War.
While we were in (on?) Guernsey we paid a visit to the German Occupation Museum. Since Guernsey is such a small Island, and the Germans were there for some 5 years (and left an awful lot of stuff behind when they left), there is an enormous amount to see.
The Museum doesn't have its own website, but it does have a review page which you can read here.
Well worth a visit if you ever find yourself in that neck of the woods.