The ornamental design for a postbase, as shown and described.
FIG. 1 is a front perspective view of a post base showing our new design;
FIG. 2 is a top view thereof;
FIG. 3 is a side view thereof; and,
FIG. 4 is a bottom view thereof.
Left, right, and rear side views of the post base are omitted as they are symmetric in appearance to the side view of FIG. 3.
The broken lines in the drawing depict environmental subject matter only and form no part of the claimed design.
The broken away symbols in the drawings indicate the appearance of any portion of the article between the break lines forms no part of the claimed design.