I had one of these on my No.1 converted to 7x57r. They have two grub screws on the bridge end that you wind out to wedge the mount between the bridge and the front receiver ring. They need a bit of shaping to get a good full contact to the mating faces.
After about twenty rounds the thing jarred itself off and smacked me in the face with the scope!
You can guess my opinion of them....

Roger

....the continuing evolution of my 7x57r...