Sheaffer introduced the Touchdown filling system in 1949. This was a unique filling system using pneumatic air pressure in the down stroke of the cylindrical plunger to compress the sac inside the cylinder and fill the pen. A similar system was employed by Chilton in their pneumatic self-filling pens introduced in 1924.
How It Works
When the plunger is pushed back into the barrel, the air pressure generated compresses the ink sac, expelling its contents. As the sac reinflates, if the nib is inserted into ink, it will fill with ink. On early Touchdowns only one down stroke is necessary to fill the pen. Interestingly, on the modern Touchdown Legacy, Sheaffer describes the pen needing two downstrokes to fill. As the pen is a cartridge / converter pen with a removable Touchdown sac unit, it is likely that the modified Touchdown system is not air tight enough to generate enough pressure to fully expel and inflate the sac, needing two strokes to do the job.