These are fitted to most of the 1950s Parker models.  To fill you unscrew the barrel, dip the nib in ink then press the pressure bar fully with your thumb or thumb nail.  Release the pressure bar and wait for the ink to be drawn in.  The aerometric filler has a type of pump action, so it is necessary to repeat this about 5 times, or until pressing the bar stops producing air bubbles in the ink bottle.  Finally, gently shake off surplus and clean the nib with a tissue.