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