The ornamental design for a dummyplug for a modular jack, as shown and described.
FIG. 1 is a bottom perspective view of a dummy plug for a modular jack, showing my new design;
FIG. 2 is a top perspective view thereof;
FIG. 3 is a bottom plan view thereof;
FIG. 4 is a top plan view thereof;
FIG. 5 is a right side elevation thereof; the left side elevation being a mirror image thereto;
FIG. 6 is a rear elevation view thereof; and,
FIG. 7 is a front elevation view thereof.
The broken line showing of the modular jack in FIG. 1 is included to show unclaimed environmental subject matter only and forms no part of the claimed design.