This is the Wm. Malcolm Shiloh Front Ring and Dovetail Set. This is designed to be used with the Wm. Malcolm 6X Long Telescopic Sight.
Some of today's rifles (Shiloh, C Sharps Arms) feature larger front and rear dovetail cuts in their barrels.
The front ring goes into the 0.453" dovetail. The rear block goes into the 0.486" dovetail.
