a reference device for cooperating with {HEAD ~ the cylindrical part} in orbital motion and defining {TAIL ~ a checking condition} of said apparatus 
a reference device for cooperating with { HEAD ~ the cylindrical part } in orbital motion and defining { TAIL ~ a checking condition } of said apparatus
OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH