The ornamental design of an adjustable concavecoverplate, as shown and described.
FIG. 1 is a front perspective view of the adjustable concave cover plate in accordance with the present invention, showing the adjustable concave cover plate mounted on a concave grate; the environment comprising a concave grate is shown in dashed lines for illustrative purposes only.
FIG. 2 is a second front perspective view thereof;
FIG. 3 is a right side view thereof in conformance with the curvature of the environment;
FIG. 4 is a left side view thereof;
FIG. 5 is a right end view thereof;
FIG. 6 is left end view thereof;
FIG. 7 is a top view thereof; and,
FIG. 8 is a bottom view thereof.
The broken line lines are shown in the views for illustrative purposes only and form no part of the claimed design.