The ornamental design for an external cornerstrip, as shown and described.
FIG. 1 is a lower right perspective view of an external corner strip embodying our new design;
FIG. 2 is a bottom view of the external corner strip of FIG. 1, the top being a mirror image thereof;
FIG. 3 is a front view of the external corner strip of FIG. 1, the right side being a mirror image thereof; and,
FIG. 4 is a left side view of the external corner strip of FIG. 1, the back being a mirror image thereof.
The drawings contain a symbolic break indicated in dot-dash broken lines. The portion between the break lines forms no part of the claimed design. The dash-dash broken lines illustrate portions of the external corner strip that form no part of the claimed design.