A registration wall may be positioned beside the entrance pulleys to register {HEAD ~ upper edges} of {TAIL ~ the envelopes} 
of
A registration wall may be positioned beside the entrance pulleys to register { HEAD ~ upper edges } of { TAIL ~ the envelopes }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL