The ornamental design for a window portion of a stampless envelope, as shown and described.
FIG. 1 is a front elevational view of a window portion of a stampless envelope showing our new design, the remaining portions of the envelope including the additional window being shown for environmental purposes only and forming no part of the claimed design; and,
FIG. 2 is a front elevational view of a window portion of a stampless envelope, the remaining portions of the envelope including additional windows in an alternative placement being shown for environmental purposes only and forming no part of the claimed design.
The broken lines defining the boundaries and the FIM code within the claimed area in FIGS. 1 and 2 are shown for illustrative purposes only and form no part of the claimed design.