The ornamental design for a “pen for a printer”, as shown and described.
FIG. 1 is a front, right perspective view of a pen for a printer.
FIG. 2 is a right side view of the pen for a printer of FIG. 1.
FIG. 3 is a left side view of the pen for a printer of FIG. 1.
FIG. 4 is a rear view of the pen for a printer of FIG. 1.
FIG. 5 is a front view of the pen for a printer of FIG. 1.
FIG. 6 is a top view of the pen for a printer of FIG. 1; and,
FIG. 7 is a bottom view of the pen for a printer of FIG. 1.
The broken lines in the drawings illustrating portions of the pen for a printer and surfaces next to the broken lines form no part of the claimed design.