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