The ornamental design for electric power transmission line support tower, as shown and described.
FIG. 1 is a perspective view of an electric power transmission line support showing our new design;
FIG. 2 is a front elevational view thereof, the rear elevational view being a mirror image;
FIG. 3 is a right side elevational view thereof, the left side elevational view being a mirror image;
FIG. 4 is a top plan view thereof; and,
FIG. 5 is a bottom plan view thereof.
The broken lines in the drawing figures are provided only to indicate the environment in which the design is associated, and form no part of the claimed design.