An assembly at a sewing machine for forming a piped-edge or bound pocket opening in a garment workpiece includes a pair of sewing needles for stitching two parallel seams in the workpiece and in a reinforcing strip laid thereon, a blade for cutting a slit in the fabric pieces between the two seams and a pair of cross-sectionally angular cutters removably clamped to the plunger of a pneumatic actuating cylinder and having vertices engaging opposite sides of a mandrel also mounted on the plunger. Upon charging of the cylinder, the mandrel passes through an end of the slit to widen same and consequently enable the traversing of the slit by one or the other of the angular cutters, whereby one cutter is effective to form a V-shaped incision in the fabric pieces at the slit end and the other cutter is ineffective.