What is this instrument that has "PACK" written on it and a temperature scale?
It looks like the pack temperature gauge for the air conditioning system on the 727


  To monitor the operation of the air cycle machines, each has a
  temperature transmitter at the outlet off  its compressor.  The
  temperature sensed is displayed on a pack temperature gauge for each
  pack.  As more air is directed through cycle machine to provide more
  cooling, more compression is  required from the compressor.  This
  results in a higher compressor outlet temperature.  Therefore, the
  pack temperature gauge monitors air cycle machine work load.  To
  protect the compressor from excessively high temperature an over
  temperature sensor at the outlet of the compressor will cause the pack
  to shut down if the temperature reaches the limiting value.  Another
  temperature limiting sensor located at the inlet to the turbine.


You can find a full video of how that section of the flight engineer panel works here.


Here it is on the flight engineer's panel (airliners.net).