The ornamental design for a socketadaptor for squarenut, as shown and described.
FIG. 1 is an isometric view of a socket adaptor for square nut according to the present design;
FIG. 2 is a side view thereof;
FIG. 3 is a top view thereof;
FIG. 4 is a bottom view thereof;
FIG. 5 in an isometric view thereof in a use environment (attached to a handle and driving a nut).
The broken lines shown in FIGS. 1-5 relating to an opening for receiving a tool, and in FIG. 5 illustrate a use environment of the socket adaptor for square nut, and do not form part of the claimed design.