If you have a desk, you need a desk pen. I try to avoid words such as need. In this case it is appropriate. The unfortunate thing is that you may not realize how much you need a desk pen until you have one. Then you will wonder how you managed without.
1949 was a singular year for Sheaffer pens. The Triumph was replaced with the Touchdown - so named for its filling system. The Touchdown lasted for only ~1 year. In 1950 it was replaced with the Touchdown Thin Model (TD TM). The TD TM is the same pen but thinner in diameter.
The Touchdown filler is a pneumatic system with a sac. I liken it to a bicycle pump. The filler is a hollow, steel tube surrounding the sac. It has a clever valve comprised of an o-ring, a dimple in the filler tube and a pin hole in the barrel wall. Pushing the tube into the barrel compresses the air inside the barrel and thus compresses the sac, purging it of air. When fully closed the valve allows air into the barrel, the sac expands and fills with ink. Simple, reliable and robust.
The pen is in very good condition. There are no cracks, blemishes or faults of any kind. The sac has been replaced with one of silicone. Silicone will last longer than latex and allows the use of any fountain pen ink. There are a couple of minor faults with the pen socket. Two "flea bite" marks above the Sheaffer'S imprint and a stress fracture near the lip. The fracture does not reach the lip and it should not propagate into a full break since the socket is equipped with a metal thread ring that bears any stress. The pen threads into the socket to provide an air tight seal keeping the pen ready to write at all times. The corners of the Mexican onyx base are rough but there are no other faults in the base.
The nib has been tuned to write a pleasant, smooth, wet, fine-medium line that is equivalent to a modern western fine.