I claim the ornamental design for the flexible sheet sign substrate with spring loop support, as shown and described.
FIG. 1 is a front view of the flexible sheet sign substrate with spring loop support in a fully deployed state.
FIG. 2 is a rear view of the flexible sheet sign substrate in a fully deployed state.
FIG. 3 is a top view of the flexible sheet sign substrate in a fully deployed state.
FIG. 4 is a bottom view of the flexible sheet sign substrate in a fully deployed state.
FIG. 5 is a right side view of the flexible sheet sign substrate in a fully deployed state (the left side view being a mirror image of the right side view).
FIG. 6 is a front view of the flexible sheet sign substrate with spring loop support in an alternate collapsed state.
FIG. 7 is a top view of the flexible sheet sign substrate in a collapsed state.
FIG. 8 is a rear view of the flexible sheet sign substrate in a collapsed state.
FIG. 9 is a bottom view of the flexible sheet sign substrate in a collapsed state.
FIG. 10 is a right side view of the flexible sheet sign substrate in a collapsed state; and,
FIG. 11 is a left side view of the flexible sheet sign substrate in a collapsed state.
In the drawings, the broken lines represent stitching forming part of the claimed design.