The ornamental design for a holderarm for mannequin's head, as shown and described.
FIG. 1 is a perspective view of the holder arm of the subject invention, with a mannequin's head attached thereto and shown in dotted lines;
FIG. 2 is a perspective view of the holder arm of the subject invention as mounted to a wall;
FIG. 3 is a side elevational view of the holder arm of the subject invention taken from the left side of FIG. 2;
FIG. 4 is a top plan view of the holder arm of the subject invention;
FIG. 5 is a rear elevational view of the holder arm of the subject invention;
FIG. 6 is a front elevational view of the holder arm of the subject invention; and,
FIG. 7 is a bottom plan view of the holder arm of the subject invention.
The broken line showing the mannequin's head and the wall to which the holder arm of the subject invention is attached is for illustrative purposes only and forms no part of the claimed design.