This pen uses the barrel as the reservoir. At the end is a small plunger under the blind cap. Attached is a short sac, half folded over on itself. Pressing the plunger pushes the sac farther into the barrel to expel a small amount of air. You have to press and release about 10 times to get the pen to fill. Video below.
In the interest of hair splitting, this came in 3 versions, lockdown, speedline, and the plastic plunger shows below. These are well illustrated at David Nishimura’s site Vintage Fountain Pens Filling Instructions: Parker Vacumatic Filler.
I will summarize for those too lazy to click the link.
The speedine is the same as the plastic plunger in terms of function, but the plunger is a hollow aluminum tube that contains the return spring, which is a compression spring, compressed when the plunger is pressed. The spring expands to return the aluminum shaft tube to the extended state.
The lockdown version is the oldest version. When the aluminum plunger is pressed in, a slight rotation locks it in the compressed position. This allows for a shorter blind cap. However, the thin aluminum of the plunger tube can be damaged at the point that functions to lock it down.
The plastic vacumatic plunger rod style hides the spring inside the collar that screws into the barrel, and the spring is an extension spring that is stretched inward when the plunger is pressed.
