{HEAD ~ an indicator means} rigidly affixed to {TAIL ~ the swing arm} for visually indicating the variation of the thickness of a brake block from a specified thickness 
rigidly affixed to
{ HEAD ~ an indicator means } rigidly affixed to { TAIL ~ the swing arm } for visually indicating the variation of the thickness of a brake block from a specified thickness
HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH