The ornamental design for an aftertreatmentdevice for an exhaust treatment system, as shown and described.
FIG. 1 is a left side elevation view of the aftertreatment device for an exhaust treatment system;
FIG. 2 is a right side elevation view thereof;
FIG. 3 is a rear elevation view thereof;
FIG. 4 is a front elevation view thereof;
FIG. 5 is a top plan view thereof;
FIG. 6 is a bottom plan view thereof; and,
FIG. 7 is a perspective view thereof.
The broken line showing of parts of the drawings is included for the purpose of illustrating use and environment and forms no part of the claimed design.