This clamp is made of sturdy plastic with 2 - 4 5/8" rubber suction cups, a special turn buckle that can span 5" for tightening and has an adjustment wedge to level the seam for less sanding.  It is used for the jobs too small for the SR4X model or to work in conjunction with the SR4X.  If used solely independently a minimum of (4) SR2X clamps is suggested to be used on a average 25" size seam.