The ornamental design for a portion of a keyboard, as shown and described.
FIG. 1 is a perspective view of a portion of a keyboard showing our new design;
FIG. 2 is a perspective view of the design with an area shown in dashed lines;
FIG. 3 is an enlarged view of the area shown in dashed lines in FIG. 2;
FIG. 4 is a top view of the design;
FIG. 5 is a front view thereof;
FIG. 6 is a left side view thereof;
FIG. 7 is a right side view thereof;
FIG. 8 is a rear view thereof; and,
FIG. 9 is a bottom view thereof.
The broken line showing of the remainder of the keyboard is included for the purpose of illustrating unclaimed portions of the article and forms no part of the claimed design.