The ornamental design for a findertube for a surgical instrument, as shown and described.
FIG. 1 is a perspective view of a finder tube for a surgical instrument, comprising the new design;
FIG. 2 is a left side elevational view of the design of FIG. 1, where the right side elevational view is a mirror image thereof;
FIG. 3 is a top view of the design of FIG. 1;
FIG. 4 is a bottom view of the design of FIG. 1;
FIG. 5 is a front elevational view of the design of FIG. 1; and,
FIG. 6 is a back elevational view of the design of FIG. 1.
The broken lines immediately adjacent to the claimed areas represent the bounds of the claimed design while all other broken lines are directed to the environment and are shown for illustrative purposes only; the broken lines form no part of the claimed design.