a pair of {HEAD ~ arcuate portions} interconnecting opposed side edges of {TAIL ~ said flat portions}, respectively, one of said arcuate portions being formed with a pair of engage grooves on an outer surface thereof so as to extend longitudinally of said arcuate portion, and 
a pair of { HEAD ~ arcuate portions } interconnecting opposed side edges of { TAIL ~ said flat portions } , respectively , one of said arcuate portions being formed with a pair of engage grooves on an outer surface thereof so as to extend longitudinally of said arcuate portion , and
OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH TAIL 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 OTH OTH OTH OTH OTH OTH OTH OTH OTH