{HEAD ~ a single block} composed of catalyzed acrylic resin and having {TAIL ~ a pair} of prism boxes inversely positioned and interconnected by a pair of posterior walls provided with a window, 
having
{ HEAD ~ a single block } composed of catalyzed acrylic resin and having { TAIL ~ a pair } of prism boxes inversely positioned and interconnected by a pair of posterior walls provided with a window ,
HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH