The ornamental design for an endcap for a paper tube, as shown and described.
FIG. 1 is a perspective view of an end cap for a paper tube showing my new design.
FIG. 2 is a top view of the end cap of FIG. 1.
FIG. 3 is a front side view of the end cap of FIG. 1, with the right, back and left side views being identical to the front side view; and,
FIG. 4 is a bottom view of the end cap of FIG. 1.
The matter shown in broken lines in the figures is present solely to indicate an exemplary perimeter rim of the end cap, and does not form part of the design.