On original BB installs, the driver side mount attaches with three bolts, two at the front of the block and one at the side of the block as in your picture. These blocks were leftover "B" blocks that had been modified by cutting off the two bolt hole tabs on the side of the block to eliminate these in favor of using the more forward "3" for "A" bodies. It should be easy to see the cut off tab on the side of the block and lack of the 3 hole side mounting.
