Filling system
Sheaffer's Touchdown (1949) fills with air pressure. Pull out the tube at the back, push it down, and the pressure squeezes a sac inside the barrel. The Snorkel (1952) adds a thin tube that slides out from under the nib, so you fill from the bottle without dipping the nib at all. The PFM of 1959 kept the Snorkel filler in a bigger body. They're clever, and they have more seals than any other vintage filler. Unrestored pens nearly always need a new sac, and Snorkels also need the small seals that make the tube airtight. Ask whether the pen has been restored and whether the tube extends and retracts smoothly.
4 pens we cover.

Sheaffer · 1952–1959
Snorkel and TouchdownPalladium silver Triumph point, plastic cap
Post-war golden age
$75–$170 typical ask
10 on eBay

Sheaffer · 1952–1959
Snorkel and TouchdownGold-filled cap, 14K two-tone Triumph point
Post-war golden age
$180–$250 typical ask
4 on eBay

Sheaffer · 1959–1968
Snorkel and TouchdownPlastic cap, gold-filled band, 14K inlaid nib
Post-war golden age
$260–$320 typical ask
6 on eBay

Sheaffer · 1949
Snorkel and Touchdown1949 only, Touchdown filler
Post-war golden age
$85–$210 typical ask
15 on eBay