The ornamental design for a lightbulb or portion thereof, as shown and described.
FIG. 1 is a front, top perspective view of a light bulb or portion thereof;
FIG. 2 is a top plan view thereof;
FIG. 3 is a bottom plan view thereof;
FIG. 4 is a left-side elevation view thereof;
FIG. 5 is a right-side elevation view thereof;
FIG. 6 is a back elevation view thereof;
FIG. 7 is a front elevation view thereof; and,
FIG. 8 is an enlarged view of an area marked 8 in FIG. 2.
The dashed broken lines depict portions of the light bulb or portion thereof that form no part of the claimed design. The dot-dashed broken lines represent boundaries of the claimed light bulb or portion thereof and form no part of the claimed design.