FIG. 1 is a perspective view of a string light source unit showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a back view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is an enlarged top view thereof;
FIG. 7 is an enlarged bottom view thereof; and,
FIG. 8 is an enlarged view of the encircled area 8 of FIG. 4.
The dot-dash broken lines framing portions of the claimed design are included for the purpose of illustrating enlargements, and form no part of the claimed design. The portions of the string light source unit shown in broken lines form no part of the claimed design.