As Lou mentioned, it is likely an aiming rest, a training aid in for teaching recruits how to use the sights on the rifle.
An optical accessory was mounted on the rifle behind the rear sight which had a reflective glass which allowed both the instructor standing to the side and the recruit looking down the sights to see the sight picture and to adjust the rifle aim at the target.
I have a couple of these that I use for sighting in rifles, but not a shooting rest by any stretch of the imagination. One that I have is for the Lee Enfield, the other has been modified for mounting the FN rifle.
The spotting scope provision sounds interesting, not something that I have seen before. Could you post a pic?
Mine are both WWII dated and Canadian marked. Value? Lou is in the ballpark.