I claim the ornamental design for a portion of a retaining block, as shown and described.
FIG. 1 is a perspective view of a portion of a retaining block, as viewed from the front, top, and one side of the block.
FIG. 2 is a perspective view of the retaining block portion of FIG. 1, as viewed from the rear, top, and one side of the block.
FIG. 3 is a bottom plan view of the retaining block portion of FIG. 1.
FIG. 4 is a top plan view of the retaining block portion of FIG. 1.
FIG. 5 is a side view of the retaining block portion of FIG. 1.
FIG. 6 is rear view of the retaining block portion of FIG. 1; and,
FIG. 7 is a front view of the retaining block portion of FIG. 1.
The broken lines in the drawings that are immediately adjacent to the shaded areas represent an unclaimed boundary of the claimed design. The broken lines showing the remainder of the retaining block are for illustrative purposes only and form no part of the claimed design.