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