two of {HEAD ~ said plates} are insertable in opposite directions into {TAIL ~ said entrances}, respectively, in said flat sleeve 
are insertable into
two of { HEAD ~ said plates } are insertable in opposite directions into { TAIL ~ said entrances } , respectively , in said flat sleeve
OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL REL OTH OTH OTH REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH