To elaborate on some of the other poster's comments, the square bridge was designed specifically to accommodate the scope mounts of the period, which was the German claw mount. These mounts required relatively deep cuts in the receiver ring and bridge. The double square bridge provided extra material to allow these cuts to be made, along with a nice flat surface to line up the rings, and Mauser provided them either pre-cut to take Mauser mounts, or uncut to be done by a private gunsmith later. As such, they were probably the first type of integral scope mount, however, unlike later integral mounts such as the Brno/CZ, Sako or Ruger, they were not standardized.
As others have said, their desirability and expense is a combination of rarity (in the original Oberndorf Mausers) and aesthetic appeal.
