The disc 68 has {TAIL ~ a slightly smaller diameter} than the distance between {HEAD ~ the two legs} so that it guides either one or the other leg 52 
The disc 68 has { TAIL ~ a slightly smaller diameter } than the distance between { HEAD ~ the two legs } so that it guides either one or the other leg 52
OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH