The ornamental design for “attachment ring”, as shown and described.
FIG. 1 is a front perspective view of an attachment ring showing my new design. The rear of the attachment ring has the same appearance as the front;
FIG. 2 is a top plan view of the attachment ring illustrated by FIG. 1;
FIG. 3 is a front elevational view of the attachment ring illustrated by FIG. 1. The rear of the attachment ring has the same appearance as the front; and,
FIG. 4 is a right side elevational view of the attachment ring illustrated by FIG. 1. The left side of the attachment ring has the same appearance as the right side.
The broken lines are for illustrative purposes only and form no part of the claim.