Further, a diameter D20 of {TAIL ~ the third circular disc portion} 20A of the third roller 20 is slightly smaller than a distance L31C between third collar plates 34A and the depth T20 is slightly smaller than a distance T31 between the third plate 33 and a third parallel plate 34B of {HEAD ~ the movable rail} 31 
Further , a diameter D20 of { TAIL ~ the third circular disc portion } 20A of the third roller 20 is slightly smaller than a distance L31C between third collar plates 34A and the depth T20 is slightly smaller than a distance T31 between the third plate 33 and a third parallel plate 34B of { HEAD ~ the movable rail } 31
OTH OTH OTH OTH OTH OTH TAIL TAIL 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 OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH