The ornamental design for a cufflink, as shown and described.
FIG. 1 is a perspective view of the invention;
FIG. 2 is a second perspective view of the invention;
FIG. 3 is a top view of the invention;
FIG. 4 is a front view of the invention, with the rear view being the same; and,
FIG. 5 is a side view of the invention, with the other side view being the same.
Please note that the particular visual features of the cufflink that are new and distinctive are indicated by the features in solid lines in the figures. The features shown by dashed lines in the figures are to be disregarded when construing visual features of the design.
The broken line showing of environment in the drawings is included for the purpose of illustrating only and forms no part of the claimed design.