An ornamentaldesign for a guitar fretboard for a six-string guitar, as shown and described.
FIG. 1 is a perspective view of a guitar fretboard for a six-string guitar, showing my new design in an exemplary environment.
FIG. 2 is a side view thereof;
FIG. 3 is a top view thereof; and,
FIG. 4 is a second side view thereof.
The broken lines are included for the purpose of illustrating the environmental structure and form no part of the claimed design. The light and dark shading of the guitar fretboard represents a contrasting appearance.