"C" style mounting is found on some W type blocks, and it provides the versatility of mounting either from the top or the bottom.
In this style, each hole is tapped near the top of the block, but there is also a counterbore on the bottom. Thus, a cap screw can be passed through the work and secured directly into the block from the top. Alternately, a slightly smaller cap screw can be inserted from the bottom through the hole to secure the work from this direction.
You can get more specific information about hole sizes and recommended fasteners in the various bearing tech pages in this site.