It is noted that {HEAD ~ the underside} of {TAIL ~ first board} 16 cannot be seen in FIG 1, but it is understood that its recess area is identical to second board 18 
It is noted that { HEAD ~ the underside } of { TAIL ~ first board } 16 can not be seen in FIG 1 , but it is understood that its recess area is identical to second board 18
OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD 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 OTH OTH OTH OTH OTH