I have heard about these for years but have never seen one in person. Brady had one at one point in the past but I have never seen it. The bridge mount was meant for you to mount a Unertl or external adjustable scope like a Unertl to the receiver of your rifle without having to have your barrel drilled and tapped for the front scope mount while maintaing the proper spacing for a true 1/4 inch per click windage and elevation adjustment. The one pictured below made for the Unertl POSA mounts.

The fellow below posted the info on the Unertl collectors group. He reported hating the bridge mount and regretted buying it back in the day. My friend had the same experience